A HOME AND SMALL BUSINESS PHONE SYSTEM FOR OPERATION ON A SINGLE INTERNAL TWISTED PAIR LINE

Abstract: The invention is a telephone communication system for communication between a plurality of exterior telephone lines (12) and a single common in house two wire line (20). The system comprises a control unit (10) for coupling to the plurality of exterior telephone lines (12) and for controlling communication between the plurality of exterior telephone lines (12) and the common two wire line (20). A plurality of station units (18) are coupled remotely throughout the building to the common two wire line (20). The control unit (10) communicates on the digital channel with the plurality of station units (18) in a time frame subdivided into a plurality of time slots. A specified portion of each time slot is reserved for control communication between the control unit (10) and each one of the plurality of station units (18).

Description: A HOME AND SMALL BUSINESS PHONE SYSTEM FOR OPERATION ON A SINGLE INTERNAL TWISTED PAIR LINE 5 Background of the Invention 1. Field of the Invention The invention relates to the field of telephone systems and in particular telephone systems in which multiple stations may be operated on a single shar 0 telephone line. 2. Description of the Prior Art The System Home and small office telephone systems are typically purchased and operat 5 by the telephone user. In addition, the telephone user or the owner of the building which the telephone system is installed, is similarly responsible for the installation a maintenance of the internal telephone lines beginning at the telephone company li terminal at the entry point to the building. Very commonly, the internal telepho wiring which is available in a home or small office is a single line system. In oth 0 words, while there may be multiple telephone jacks, each of the jacks are coupled parallel to a single telephone line within the structure. Therefore, only one telepho conversation at a time may be carried on the line and if two handsets are picked up, t power on the line is distributed between the two activated handsets with a conseque loss of audible volume. Additionally, the communication as well as the central offi 5 link must be generally held in common between the two participants. A number of schemes have been attempted for expanding the communicati capability of a single internal telephone line network, most of which involved vario voice band multiplexing schemes. As the features and complexity of the telepho network increases, greater demand is placed upon the multiplexing system. T 0 complexity of the multiplexing scheme can become prohibitive when the system must full featured. For example, when the system must provide an automated operator provide voice message answering of incoming calls, special handling of fax, pho modem and answering machines at the telephone stations, extension dialing, a host separate business phone features and fax features, and accommodate a large number 5 phone stations all incorporated in an economical package which easily lends itself expansion and modification, the prior art has failed to provide practical solutions. T difficulty arises from many reasons including the following: (1) The expense and labor involved in wiring or rewiring the facility if a star wire system is used (a separate wire to each phone); and (2) the complexity and technical difficulty in implementing a digital or frequency band (AM, FM, etc.) form of sharing a single common line. Finally, the high communication and computer overhead time which is required in a multiplexing system to handle multiple features and stations with complete flexibility and adaptability. The amount of digital communication which must be carried on often becomes prohibitive and the system fails during peak periods or unusual demand scenarios. Many multiple wire (star wire) as well as some two-wire small phone systems utilizing a master controller and multiple station controllers are known in the art. The multitude of star wire systems suffer from both the inherent wiring complexity as well as the following problem. One typical two wire system is the Model 8002 MCD base key telephone system unsuccessfully offered by Rockwell. In such systems, a sophisticated processor is not only required in the master controller, but also at each station which adds to the cost and complexity of the system. An architecture which is built upon multiprocessing makes changes in the system difficult since modifications must be made in both the controller program and in the station programs. Communication between these multiprocessors is complex and when the number of stations increases, overall communication can become very slow during busy periods. Only a few features can be changed remotely since again most of the features require changes in both station and controller programs. In the Rockwell system, a mix of voice channels, digital channels and a reference signal on a twisted pair telephone line, using double side band suppressed carrier with amplitude modulated signals was employed. However, the Rockwell system did not define the structure of digital data communication, nor did it remove the requirement for a separate control processor in every station. Therefore, although it did allow multiple signals on a single twisted pair telephone line, it did not provide any simplification to the complexities of multiprocessing nor solve the inflexibihties inherent in multiprocessing architectures. Additionally, this product suffered from severe noise, phase lock, and synchronization problems which eventually doomed it to failure. Therefore, what is needed is a single twisted pair, multichannel telephone system which can be economically and easily adapted to a home or small business and which has the power and flexibility to perform the functions discussed above without undue complexity, expense or susceptibility to failure under heavy demand or unusual use scenarios. Additionally, it needs to solve the critical noise and synchronization problems inherent in such a system without resort to a full digital (and very expensive) solution. Simplified Processor Conventional general purpose processors typically operate on a program stored in a read only memory by means of an instruction counter in order to read the stored instructions in sequence or according to a sequence with programmed jumps. This architecture is relatively complex, requires large numbers of transistors to implement and will therefore use a large area of a chip when integrating this function, and is time consuming of real time communication cycle time. The disadvantages of such a processor are particularly burdensome in an application where general programmability is not required. What is needed is an architecture and method of operation for a processor which is more usable for applications that require only a limited number of operations and that avoids the overhead and timing disadvantages of a general purpose processor. Communication Signaling Scheme In a typical prior art small telephone system, a separate pair of wires is coupled from a control unit or master phone to each remote station or telephone handset. This type of system is commonly referred to as a "star wire" system. The control unit determines where the message is coming from and where it will be going according to which pair of wires is selected. This system has the disadvantage that there is a need to provide a separate wire pair for each remote station connected to the control unit and thus eliminates the use of standard house telephone wiring since conventional home wiring typically connects to all the extensions or telephone stations in parallel on a single pair of wires. The prior art has also devised a scheme in which a single pair of wires is shared between multiple remote stations connected to a control unit. The control unit or master phone manages the telephone network by using a message based protocol. Whenever a phone call or message is to be sent, the transmitting unit, regardless of whether it is a remote station or control unit, will initiate a message in the network by sending an initial data protocol which will define the transmitter and recipient of the message. This type of message based protocol is subject to slow downs or lock up as the number of remote units and systems activity increases. What is needed is a communication protocol for a small telephone system which is not subject to the limitations of the prior art. In particular, the protocol should eliminate the need in the system to establish a handshake protocol every time a message is sent in either direction to avoid slow downs during active communication periods between a plurality of units. Such a protocol should also allow serial digital data to be transmitted over the same line in burst format without affecting signaling speed so that components such as a display can be serviced quickly without affecting signaling response time. Additionally, a good error correction scheme must be implemented without affecting signal response time. Serial data should be quickly transferred for display or use in other serial communication network applications regardless of the direction of transfer. Voltage Controlled Crystal Oscillator Voltage controlled crystal oscillators in the prior art generally do not use MOSFETs to modulate the output impedance of the amplifier stages in the oscillator to shift the phase of the output in order to change the frequency of the crystal controlled oscillator. In addition, prior art voltage control oscillators are generally not designed to be easily integrated in large communication circuits. What is needed is a crystal controlled voltage oscillator using phase shift techniques in which the frequency of oscillation can be adjusted within a tight and stable range and which has a topology suited to integrated circuitry and that utilizes a small chip area. Light Emitting Diode Driver Circuit The standard method for driving multiple Hght emitting diodes (LEDs) is to drive them in parallel through a series resistance and switched by appropriate logic signals. The current through any hght emitting diode is determined by the value of its corresponding series resistor when the logic switch is closed which is also in series with the diode. When all the Hght emitting diodes are on, the current is additive and may be substantial depending upon the number of LEDs. Therefore, what is needed is a circuit in which the total current used to drive a bank of Hght emitting diodes can be held constant and limited for lower power appHcations. Further, if the mechanism using the diodes is line powered on a line also used for communication the amount of noise which switching of the current through the Hght emitting diodes places upon that communication line should be avoided as much as possible as it could interfere with that communication. In addition, the current suppHed to each LED must be maintained as constant as possible in order to maintain consistent LED brightness within a multiple bank of LEDs regardless of how may of the LEDs may be Ut. These attributes are difficult to maintain with parallel LED switching. Automatic Timing Compensation for a Communication Line Telephone systems which use an internal telephone cable and which carry voic and/or data modulated signals at high frequencies are characterized by line dela between signals transmitted between the control unit and the station unit and betwee two station units. In order to compensate for these inherent line delays, the prior art h devised circuits for advancing the transmission signal or delaying the received signal fixed average amount given the line length specification variations if the Hne delay above a predetermined minimum and otherwise accepting smaller line del inaccuracies. Another method used in the prior art is to manuaUy adjust the compensatio components tied to the line at the time of instaUation in order to attempt to cancel o line delay variations. What is needed is some type of circuit which provides for automatic adjustme to the transmission signal to avoid line delays , a circuit which can be implemented low cost and which adjusts for potential load changes such as may occur when a ne station is plugged in. A Low Cost Adaptive Echo Balance Methodology Whenever there are four-to-two wire conversions in a telephone system, there an echo back of the transmitted signal which must be given consideration in the desig of the system. In such appHcations where echo in not critical, the provision of a fixe component balance network is usually a satisfactory solution. In other appHcatio where echo canceUation is more critical, normaUy a signal processor particular adapted to cancel the echo is employed. What is needed is some type of apparatus and methodology wherein ech cancellation can be achieved in appHcations where a fixed network would not provide satisfactory solution, but which does not require the more expensive compensatio based upon a digital signal processor for echo canceUation. A Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled, Communication Line Interface A typical prior art means for interfacing multiple signals onto an internal (insid the house or office) twisted pair communication line is comprised of a transform across the primary of which the internal communication line is connected and across th secondary of which a " transmit buffer and receive buffer are provided for bidirection communication with internal circuitry. The internal communication line could be telephone cable within a home or office and the internal circuitry to which it would be coupled would be a key telephone circuit or handset. A pluraHty of such transmit and receive circuits could be coupled in paraUel across the secondary of the transformer to couple multiple sources at the same location to the internal communication line through a single interface. Each source to be coupled to the internal communication line at different locations of course requires a separate transformer and in a typical system, a multipHcity of transformers are used corresponding to the number of remote sources. The large number of transformers necessary for a useful system creates a large accumulation of magnetizing inductance, leakage inductance, core saturation and resonance which degrades the communication transmission for relatively wide bandwidth signals and thereby severely limits the number of interfaces that can be reHably coupled to the internal communication line. Therefore, what is needed is a circuit and method to provide interfacing for a two-wire twisted pair that wiU support: (1) multiple signals being communicated on the line at the same time through the same interface such as a reference frequency, digital signaling data, and/or voice modulation at different frequencies; (2) signals being communicated through several interfaces simultaneously from different locations on the line; (3) large variation in Hne load and other system loads without significant signal degradation; (4) the supplying DC power over the same lines; (5) a common low impedance Hne load; and (6) echo canceUation to eliminate false data from being transferred. Telephone Line Interface A typical telephone line interface uses a transformer to couple to the central office lines and two buffer amplifiers coupled to the secondary of the transformer to provide a signal OUT and signal IN to the equipment being interfaced whether a common telephone or a sophisticated PBX system. An example of such a prior art line interface unit is shown in Figure 21 and includes transformer 338 with a secondary 340 and primary 342. Amplifier 344 is used to generate the signal OUT and is coupled to secondary 340 whUe amplifier 346 drives the secondary from the signal IN. Summing resistors 348 associated with the transmit amplifier 344 sums the input signal and the output signal to thereby provide basic echo canceUation. Output drive amplifier 346 and its associated summing resistors 350 and output resistor 352 drive the appropriate output signal through the transformer 338 onto the telephone lines coupled to primary 342. The implementation of Figure 21 suffers from an erroneous echo signal due to reactive parameters associated with the greatly varying telephone line characteristics and transformer 338. A Piezo Driver Using Voltage Doubling and CMOS Techniques A typical prior art piezo circuit uses switched positive and/or negative sup voltages which are avaUable within the system, or alternatively some type of po supply which is coupled to the piezo driver to provide a higher voltage which is switch in when needed. This is a relatively expensive and space consuming solution to t problem of providing a higher voltage to a piezo ringer than is normaUy avaUable i telephone circuit. Therefore, what is needed is some means for applying a higher voltage to t piezo element without the need of providing an additional higher voltage power sup so that sound volume improvement can be provided in a manner compatible wi integrated circuit technology at low cost. Voltage Limiter Prior art voltage limiters for amplifiers typicaUy use cascaded diodes coupled paraUel across the input and output of the amplifier in one or both directions to limit t voltage range of the ampHfier. The linear range of such cascaded diodes is, howev limited. Therefore, what is needed is some type of circuitry in which voltage limitati across an amplifier can be achieved over an extended linear dynamic range, closer to t voltage limit points. Line Powering for Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled Communication Line Interface When remote electronic devices are powered by the communication H consideration must be given to the noise generated by providing power on the sa wires as the communication signals. In a typical house telephone line, this type of no is typicaUy not severe since aU the operating devices share the same communicati channel and the same parties to the communication. In that case, any noise synchronous with a voice conversation in progress and is generaUy not harmful. In th cases where the line is shared between devices having signals which are incompatib such as in a telephone and modem communication, the user generaUy does riot use t incompatible devices at the same time and thereby avoids the conflict. However, in t case of simultaneous, unrelated communication of multiple devices sharing the sa wire, Hne powering becomes very compHcated and the solutions are costly if low noise required. In a two-wire Hne, one wire is provided at a positive voltage and the other at negative voltage to provide a constant current flow to the remote stations. In order to energize such a system, the voltage must first be placed across the two wires of the Hne by a Hne power source. Then the remote stations must have means for puUing power from the line at a constant predictable rate in order to minimize the possibiHty of generating noise on the line which might interfere with the communication. Digital and analog circuits typicaUy utilize a constant voltage and have variable current demands. In addition, many such circuits require several different voltages to be suppHed each with different current demands at different times. It is further desirable to minimize the different types of voltage suppHes needed and to provide a current balance between the positive and negative rails as much as possible in order to avoid drift of the voltage suppHes relative to the voltages of the supplying two-wire lines. Therefore, what is needed is some sort of apparatus and methodology for line powering whereby multiple devices can employ a common line with multiple channels having unrelated signals, which devices communicate with different parties without the generation of noise through the line powering which interferes with any of the communication. Brief Summary of the Invention The System and the Chip The invention is a telephone communication system for communication between a plurality of exterior telephone lines and a single common in house two-wire Hne. The system comprises a control unit for coupling to the pluraUty of exterior telephone lines and for controlHng communication between the plurality of exterior telephone lines and the common two-wire line. A pluraUty of station units are coupled remotely throughout the buUding to the common two-wire line. Each of the station units selectively communicates on one of a pluraUty of voice bands or channels with any one of the ' exterior telephone Unes or with another station under the control of the control unit. Each of the station units communicates digital control and data information with the control unit on a separate AM modulated frequency band or channel on the common two wire line. The control unit communicates on this digital channel with the pluraUty of station units in a time frame subdivided into a pluraUty of time slots. A specified portion of each time slot is reserved for control communication between the control unit and each one of the pluraUty of station units. AU control communication between each one of the station units and control unit occur within the selected portion of the time slot within each time frame. As a result, control communication on the single common two- wire line is effected without requiring complex control hardware within the control unit or station units. Communication between the station units and the control unit is on least one AM digital channel. The control unit generates a reference frequency to which the plurality of stati units are synchronized. The plurality of station units and the control unit synchronized with each other through phase lock synchronization to the refere frequency using crystal controUed VCO. The system further comprises a pluraUty of modulating interfaces in the cont unit. Each modulating interface, contained in part of a MAN chip, services a separ one of the exterior telephone lines. One of the pluraUty of modulating interfaces ser as a master and the remaining ones are slaved to the master. The pluraUty of modulating interfaces (MAN chips) in the control unit and in t station units are identical, but operated in different modes according to their respecti operational position within the system so that a system based on a single integrat circuit is provided. The control unit also contains the central microprocessor which is programma and aU communication features between the control unit and the plurality of statio and their relationship with the exterior telephone lines are stored within this control u processor so that the entire system may be modified by modifying the programs stored the control unit. Because of this, the system may be more easUy changed and may ev be remotely modified through the exterior telephone company Unes coupled to t control unit. The control unit commumcates with the pluraUty of station units through a digi data channel. The digital data channel has two digital signals simultaneou communicated in phase quadrature through phase shifted keying. The digital data channel is fuU duplex and serial digital data is communicat through a portion of the time frame. The remaining portion of the time frame is us for communication of control information between the control unit and the pluraUty station units. The control unit and pluraUty of station units by means of their MAN chi encode, generate and communicate control and status information onto the comm two wire line over the digital data channel. The pluraUty of voice channels are simultaneously (with the above referen signal and digital data channel) communicated onto the common two wire Hne and t control unit comprises a circuit for programmably vectoring a selection of the voi channels in order to selectively, network the pluraUty of voice channels among t station units coupled to the single common two wire line. The control unit and plurality of station units by virtue of the MAN c contained within each further comprise a controUable input/output port programma capable of logic switching and sensing, keyboard and display control, as weU as tone and pulse generation. The control unit and pluraUty of station units by virtue of the MAN chip contained within each comprise circuitry for interfacing with a general purpose microprocessor for bidirectional exchange of data, control, and status information with the microprocessor. The control unit also contains circuitry to detect various tones and pulses as well as generate voice messages in order to provide for a fuU featured business telephone system capabUity, although these features are not unique of themselves. A Simplified Bus Oriented Processor Using a Fixed Time Slot Protocol The invention also includes a bus oriented processor comprising a data bus having fixed time slot access with respect to devices on the bus that are peripheral to the processor. Each peripheral device is accessed through fixed and unique portions of timing frames on the data bus. A universal logic unit is coupled to the data bus. The universal logic unit performs a predetermined assortment of operations on data read from or written to the data bus during fixed time slots within the timing frame. The processor further comprises a pluraUty of registers coupled to the data bus. Each of the registers is read from or written into during selected and fixed time slots within the timing frame. A corresponding pluraUty of combination and logic circuits are provided. Each combination and logic circuit is coupled to one of the registers and to the universal logic unit for receiving a compare/carry control signal, CMPCRY, indicative of whether or not a compare or carry occurred or not during a corresponding operation within the universal logic unit. The processor further comprises a pluraUty of bit memory circuits for storing bit flag and other machine state flags, and a corresponding pluraUty of combination and logic circuits. The corresponding combination and logic circuits are coupled to the timing and control bus. A logical combination is detected by the corresponding combination and logic circuit from the timing and control bus and the CMPRY, and the bit flag is stored within the bit memory circuit for generating a conditional logic signal. The universal logic unit performs only two basic types of instructions. The first type are compare instructions and the second type of instructions are incrementation/decrementation instructions. Each time slot is comprised of dedicated cycles. Each cycle and portion of cycle within each time slot is dedicated to bus precharge, selective data transfer from the bus or selective data transfer to the bus in a fixed sequential order so that bus voltage levels can be maintained and contention can be avoided. The invention is also a method of operating a processor to which an instructio communicated. The method comprises the function of repetitively performing sequence of timed steps. The steps include the foUowing: precharging the data b selectively reading a first data signal from the data bus during a first dedicated ti cycle depending upon the instruction; precharging the data bus; selectively readin second data signal on the data bus during a second dedicated time cycle according to t instruction; selectively performing one of a predetermined number of operations on t first and/or second data signal during the second and beginning of the third dedicat time cycle according to the instruction; selectively writing the result of the operati during the third dedicated time cycle according to the instruction; and selectiv generating a logic signal, CMPCRY, during the third dedicated time cycle according the instruction and according to the results of the step, performing a control action su as the step of writing the data into a register. The step of performing the instruction consists of one of the steps of: comparing the first and second data signals and selectively incrementing the first d signal; comparing the first and second data signals and selectively decrementing the first d signal; incrementing the first data signal; and decrementing the first data signal. Communication Signaling Scheme The invention is an improvement in a method for communicating over a sin two-wire cable with a pluraUty of stations comprising the step of bidirectiona communicating with the pluraUty of stations in a sequence of time frames. Each of t time frames is divided into a pluraUty of time slots corresponding to the pluraUty stations. Each one of the time slots is dedicated for communication to a specified one the pluraUty of stations. The step of bidirectionaUy communicating compris communicating serial digital data (that is stream format communication data) in at le one portion of the time slot and communicating signaling data (that is cont information) in at least another portion of the time slot. Each of the time frames havi a frame sync included therein to which the station units synchronize communication the two-wire cable. As a result, efficient data signaHng with the station units performed without substantial dependence on system activity. The improvement further comprises the step of communicating a reference sig to the station units from the control unit as a single timing signal against which the ti frames of each of the station units are synchronized by circuitry which senses the fra sync. The improvement further comprises the step of performing for each station unit within its corresponding dedicated time slots in consecutive frames an echo back communication protocol as determined by detected communication errors and selectively correcting the communication errors in signaling data. The signaling data is correlated in the bidirectional communication with a specified register within the station unit receiving the communication so that the bidirectional communication is easUy implemented in hardware. Each time slot is comprised of three byte cycles. Two of the cycles are dedicated to serial data and the third of the cycles is dedicated to signaling data. Each of the time slots is divided into a sequence of dedicated byte cycles. Each of the cycles is dedicated to distinguishable groups of signaHng data and serial digital data relating to communication of the control unit with the station unit. The signaling data is directed to or from separate and specific registers within the station unit. Voltage Controlled Crystal Oscillator The invention is an integrated circuit, crystal controUed voltage controUed osciUator comprising a pluraUty of phase shifting amplifiers in combination with a crystal controUed feedback loop coupled across the pluraUty of phase shifting ampHfiers. The phase shifting amplifiers are voltage controUed. A bias tracking circuit maintains a constant bias voltage within the pluraUty of phase shifting ampHfiers regardless of any control voltage appHed to the phase shifting amplifiers. As a result, control of output frequency of the voltage control osciUator is maintained over an extended range without saturation of the phase shift amplifiers. Each phase shifting amplifier is comprised of a CMOS inverter having an output node, a capacitor coupled to the output node and a voltage controUed output impedance modulating circuit for modulating the impedance of the output node and phase shift at the output node. The capacitor is a pair of CMOS shunt transistors coupled to the output node. More specificaUy the pluraUty of phase shifting amplifiers comprises three CMOS inverters. Each inverter has an output node. The CMOS inverters are coupled seriaUy with each other to form a cascaded chain of a first, second and third inverter. Two pairs of CMOS shunt transistors comprise a capacitor. One pair of the CMOS shunt transistors is coupled to each the corresponding output nodes of the first and second inverters. Two CMOS voltage controUed modulating circuits coupled to the output nodes of the first and second inverters to modulate the dynamic impedance of the output nodes of the first and second inverters to control the phase shift of the voltage at the output of the third inverter. The output of the third inverter is coupled to the crystal controUed feedback loop so that the resonant frequency of the crystal controlled feedback loop is in turn controUed by the phase shift at the output node of the thi inverter. The voltage controUed oscUlator has a fundamental frequency determined by voltage signal applied to the CMOS impedance modulating circuit. The voltage controUed osciUator is fabricated in a CMOS integrated circuit that the first and second inverters and the two corresponding CMOS impedan modulating circuits have operational characteristics dependent upon integrated circ process parameters and wherein the two corresponding CMOS shunt transistors a coupled to the corresponding first and second output nodes to produce excess phase l to cancel at least in part the effect of the integrated process parameters upon t operating characteristics. The bias tracking circuit comprises a matched dummy circuit matching operational characteristics the two CMOS output impedance modulating circuits. T dummy circuit has a dummy node. A control circuit is coupled to the control volta appHed to the voltage controUed oscUlator. The control circuit drives the dummy circ to maintain the dummy node free of displacement current as the control volta changes. The control circuit is coupled to the CMOS voltage controUed modulati circuit to essentiaUy maintain the output nodes of the first and second inverters free displacement currents as the voltage control signal changes, so that self-bias of the pha shifting amplifiers is substantiaUy independent of the control voltage. The three inverters are matched and the control circuit comprises two dum inverters, each matching the three matched inverters. An operational amplifier included. Each of the matched dummy inverters within the control circuit is coupled one of two inputs of the operational ampHfier. The dummy output node is fed back one of the inputs of the operational amplifier to maintain the dummy node at a net ze displacement current by driving the dummy circuit with the output of the operation amplifier to maintain the node at a net zero displacement current while the contr voltage is appHed to the dummy circuit. The output of the operational amplifier is a coupled to each of the CMOS voltage controUed modulating circuits to similarly dri each of the CMOS voltage controUed modulating circuits to maintain the correspondi first and second output nodes at a net zero displacement current. The invention is stUl further characterized as a method for operating a volta crystal controUed oscUlator in a CMOS integrated circuit comprising the steps receiving an input signal at an input node; and phase shifting the input signal to genera a phase shifted output at an output node by circuit of voltage controUed phase shifti circuits. The phase shifted output signal from the output node is fed back to a cryst controUed resonant circuit. The output of the crystal controUed resonant circuit is f back to the input node. Each of the prior steps is repeated to create a regenerati osciUator. The fundamental frequency of the osciUator is determined by the cryst controUed feedback circuit. The phase shift introduced at the step of phase shifting with a voltage control signal is modified to change the fundamental frequency of oscUlation. The DC bias of the phase shifting circuits used in the step of phase shifting is maintained constant as the voltage control signal changes to extend the range of voltage control adjustment of the fundamental frequency of the oscUlator without saturating the inverting ampHfiers. The step of maintaining self-bias comprises the steps of applying the voltage control signal to a dummy circuit matching the phase shifting circuit; generating a gate control signal which when appHed to the dummy circuit maintains self-bias of the dummy circuit independent of the voltage control signal; and applying the gate control signal to the voltage controUed phase shifting circuits within the voltage control osciUator. The method further comprises the step of increasing the voltage range output from the phase shifting circuits by applying CMOS shunt capacitive transistors to the output constructed of the gates of CMOS FET transistors similar to the inverting amplifier FETs themselves. The method further comprises the step of increasing the degree of phase shift during the step of phase shifting in the voltage controUed oscUlator by providing two cascaded phase shifting voltage controUed circuits driven in paraUel by the voltage control signal and coupled in series to provide an excess phase shifting lag within the voltage controlled oscillator of approximately 90 degrees. Light Emitting Diode Driver Circuit The invention is a circuit for driving a pluraUty of Ught emitting diodes comprising a series circuit of the pluraUty of Ught emitting diodes. A pluraUty of switches is provided in series circuit. Each one of the switches is coupled in paraUel across a corresponding one of the pluraUty of Ught emitting diodes so that the series circuit of Hght emitting diodes and series circuit of switches together coUectively comprise a ladder network. A constant current source is coupled in series with the ladder network. As a result, the pluraUty of Hght emitting diodes are efficiently driven with a substantiaUy constant Ught emission intensity with a reduced maximum energy. The circuit further comprises a plurahty of the series circuits of Hght emitting diodes and corresponding plurality of the series circuits of switches to form a corresponding plurality of ladder networks. Each of the ladder networks are coupled in paraUel. The circuit further comprises a gate control circuit coupled to each of the ladder networks for -selectively coupling one of the corresponding paraUel ladder networks to the constant current source. The gate control circuit selectively switches in sequence each of the ladder networks with overlapping timing so that at no time during the sequence is the constant current source every completely disconnected from at leas one of the plurahty of ladder networks. The result is that the circuit can be operated a a predetermined duty cycle for time multiplexing purposes. The invention is also a method of driving a plurahty of Hght emitting diodes usin the approach of providing a current to the pluraUty of Hght emitting diodes in series circuit. The current is shunted through a switched shunt around any selected diode tha is determined to be in a temporarily off, nonemitting condition. The current flowin through the series circuit of Hght emitting diodes and shunts is maintained constant. As a result, a pluraUty of Hght emitting diodes are driven at low power consumption as many diodes share the same current and low noise as the current remains constan regardless of the on or off state of the diodes. The approach of providing a current and shunting the current around selected ones of the Hght emitting diodes further comprises the step of providing a curren through at least one of a pluraUty of series circuits of Ught emitting diodes and controlHng the current through each one of the plurahty of series circuits of ligh emitting diodes and corresponding shunts to selectively stop and start the current whUe simultaneously maintaining current through at least one of the series circuits of LEDs at all times. As a result, the series circuits of LEDs may be driven at different duty cycles for multiplexing purposes without current spikes (and so noise) being created. Automatic Timing Compensation for a Communication Line The invention is a method for compensating for line delay variations in telephone communication system having a single shared communication line comprisin the steps of initializing communication on the single line between a control unit and at least one of a pluraUty of station units coupled by circuit of the line. This compensatio is effectively performed by maintaining two separate time bases (transmit and receive) in each communicating device and advancing transmit timing with respect to receive timing (delay receive timing with respect to transmit time). The communication is initialized to a predetermined delay value for transmit advance timing over the receive timing when the system is turned on or powered up. The control unit then performs tests on the system and the predetermined initial value for the line delay is automaticaU adjusted according to actual line delays sensed on the single communication line. As result, changes in line delay within the telephone system are automaticaU accommodated. The step of automatically adjusting the line delay further comprises the step o automaticaUy measuring capacitance effect and thereby determining approximat capacitance of the single communication line and computing a new transmit advanc time to more nearly compensate for line delay with the station units that are connected There are several types of tests that the control unit can perform in order to determine Hne capacitance and optimum advance timing. The present design injects a tone signal into one of its transmit channels and test various transmit positions and various settings of its adjustable transmit signal feedback canceUation circuitry, and monitors relative signal level on its receive signal channel. Feedback settings at specific peak and null reading positions are then appUed to a lookup table to determine optimum setting of the transmit advance timing. The step of automaticaUy adjusting the predetermined transmit advance can be performed for each of the pluraUty of station units coupled to the control unit through the single communication line. A separate automatic adjustment can be made for each station unit, or a single system wide adjustment can be made. The transmit advance timing can be set by using a static optimal adjustment by independently and optimaUy adjusting or setting the timing value in each individual communicating device (station unit or control unit) to set a fixed value depending upon its physical position on the wire and its computed best compromise timing relation with aU other communicating devices and in particular to the control unit. The transmit advance can also be adjusted by using a dynamic setting depending upon the communication task at hand for each the station. This is performed by specificaUy adjusting two particular communicating devices to specific timing adjustments prior to the start of communication between the two for the sole purpose of optimizing that one commumcation. This might be done for specific troublesome situations or possibly system wide in order to extend the wire length possible for operation or for simply better performance. However, this would be an extremely compHcated and time consuming computer task. A Low Cost Adaptive Echo Balance Methodology The invention is a circuit for reducing echo back signals on a communication line comprising a summing circuit coupled to the communication line for bidirectional communication of signals therewith. The summing circuit has a summing node. The summing node sums signals which are measures of the transmitted and received signals on the communication line. A balance network circuit couples a selected impedance to the summing node. A processor receives a signal from the summing node and generates a control signal to the balance network circuit to select an impedance to be coupled to the summing node to reduce the signal received from the summing node by the processor circuit. As a result, the echo of a transmitted signal placed on the commumcation line is substantially reduced. The balance network circuit comprises a pluraUty of selectively switche impedance elements. The switched impedance elements are comprised of a pluraUty switched capacitive elements and switched resistive elements. The processor circuit comprises a peak detector circuit for generating a consta analog voltage corresponding to an amplified signal from the summing node. A analog-to-digital converter converts the constant analog voltage to a digital signal. A analog to pulse width circuit can be substituted for the analog to digital circuit to reduc cost. A digital processor receives the digital signal and generates control signals to b coupled to the balance network circuit for altering the impedance of the balanc network circuit to m imize the echo back signal. The circuit further comprises a test tone signal generation circuit for selectivel coupling a predetermined test tone on the communication line and into the summin node to generate from the communication line the echo back signals coupled to th summing node. The circuit further comprises a station unit coupled to the communication lin The control unit communicates with the station unit across the communication Hne. Th station unit simUarly is provided with a summing circuit and balance network circui The processor circuit corresponding to the control unit communicates control signals t the balance network circuit corresponding to the station unit to substantially reduce th echo back signals at the station unit as determined by optimal reduction of the ech back signals by the processor circuit and corresponding balance network circuit at th control unit. A Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled, Communication Line Interface The invention is a system for coupling to a two-wire communication Hne to plurality of sources comprising a pluraUty of voltage-to-current ampHfiers. Eac voltage-to-current amplifier is capacitively coupled to the communication line. Th voltage-to-current amplifier bidirectionaUy communicates between the communicatio line and one of the pluraUty of sources. A circuit is included for providing hig impedance between the pluraUty of voltage-to-current amplifiers and th communication line. As a result, the pluraUty of sources are coupled to th communication line without being characterized by the mutual inductance, leakag inductance, inherent resonance, nonlinearity of magnetic media, or the windi resistance characteristic of transformer coupling. Each of the voltage-to-current amplifiers comprises two differential ampHfier Each differential ampUfier has an output capacitively coupled to one of t communication Unes comprising the two-wire communication line. One of the tw differential amplifiers is operated as a current source into ground and the other one of the differential amplifiers is operated as a current source from ground. The system further comprises an RC coupled differential receiving amplifier having two inputs. Each of the inputs is coupled to one of the two wires of the communication line. The differential amplifier has an output characterized by substantial common mode rejection of signals received from the two-wire communication line. The system further comprises a plurahty of echo balance circuits for canceling echo back signals, wherein each the RC coupled differential amplifier has its output coupled to a corresponding one of the echo balance circuits. One of the echo balance circuits is provided for each of the voltage-to-current amplifiers so that echo back signals from each source are substantiaUy reduced. The system further comprises a balanced DC load powering circuit for aUowing line powering over the commumcation line without lowering the AC impedance apparent to the line and without disturbing signal current information. The system further comprises a balance load ground circuit for setting a ground reference voltage whUe maintaining high impedance of the communication line and to aUow the plurahty of voltage-to-current amplifiers to be coupled to the communication line without significantly lowering the impedance apparent to the communication line due to the interface. Telephone Line Interface The invention is a telephone line interface for coupling central office telephone Unes with circuitry or systems such as telephones or PBX systems. The interface comprises a transformer having its input coupled to the central office telephone Unes and having an output. A load resistor is coupled to the output of the transformer and provides the proper 600 ohm termination to the central office line. An output amplifier receives an audio signal from the circuitry being interfaced to the line. An inverting amplifier is coupled to the output amplifier for inverting the output of the output amplifier and coupling the inverted output to the load resistor. An input amplifier is coupled to the output of the transformer for generating an input signal to the plurality of sources. A balance network is coupled between the output of the output amplifier and the input of the input amplifier for summing the audio signal with the inverted audio signal output through the transformer. The summing is performed at the input of the input amplifier so that echo back signals are substantiaUy reduced by canceUation. The balancing network has an impedance that simulates the central office Unes and equipment. The balance network simulates the. output impedance of the transformer as presented to the input amplifier when the transformer is coupled to a telephone central office line and equipment. The balance network can be seen as circuit for providing a source resistance equivalent to the telephone central office. Th telephone line interface thus comprises a circuit for providing an equivalent impedanc to a typical transmission telephone line. The telephone line interface therefor comprises a circuit for compensating for the magnetizing inductance of the transforme for compensating for the load resistance, for compensating for winding resistance of th transformer as it effects low frequency gain of the input amplifier, and for compensatin for low frequency cut off associated with magnetizing inductance of the transformer. A Piezo Driver Using Voltage Doubling and CMOS Techniques The invention is a circuit for driving a piezo element comprising a first CMO driver and a second CMOS driver. Each CMOS driver generates a driving voltage i response to an input control voltage. The driving voltages from the two CMOS driver are placed across the inputs to the piezo element. A circuit is provided for generatin two distinct input control voltages opposite in phase. One of the control voltages i coupled to the input of the first CMOS driver and the other one of the control voltage is coupled to the input of the second CMOS driver. As a result, an effective increase voltage swing greater than the magnitude of the supply voltages is experienced acros the first and second CMOS drivers and is appHed across the piezo element. Each of the drivers is a CMOS driver having an output equal to alternately th positive or negative supply voltages, Vp or Vn. Each of the CMOS drivers is comprise of a first stage CMOS input driver having an input coupled to the control voltage, and second stage output CMOS driver. The second stage output CMOS driver has an inpu coupled to the output of the first stage input driver. The invention is also a method for generating an improved audio output from piezo element comprising the steps of providing an input signal having a frequency a which the piezo element is to be driven. This inverted input signal is inverted. Th input signal is coupled to a first CMOS driver. An output signal is generated from th first CMOS driver in phase with the inverted input signal coupled to the first CMO driver. The generated output signal has an ampHtude equal to the difference in suppl voltage coupled across the first CMOS driver, Vp - Vn. The input signal is coupled to second CMOS driver. An output signal is generated from the second CMOS driver i phase with the input signal. The output signal generated from the second CMOS drive has a magnitude equal to the difference in voltage supply across the CMOS driver, Vp Vn. The generated output signals from the first and second CMOS drivers is couple across the piezo element. As a result, a voltage equal to substantially twice the voltag difference across the CMOS driver is appHed across the piezo element to generat increased audio volume. Voltage Limiter The invention is a circuit for voltage Hmiting the output of an amplifier having an input and output. The circuit comprises a first series circuit of bipolar transistors coupled between the input and output of the amplifier. The first series circuit of bipolar transistors is arranged and configured to limit the negative voltage swing of the output. A second series of bipolar transistors is coupled between the input and output of the amplifier. The second series of bipolar transistors is arranged and configured to limit the positive output voltage swing of the amplifier. As a result, voltage limitation of the output of the amplifier is achieved whUe maintaining high linearity within the useful dynamic range of the amplifier. The first and second series of bipolar transistors are comprised of a series circuit of a plurahty of bipolar transistors. The base and coUector of each transistor in the series circuit are coupled in common as the input of the transistor. Each of the series circuits is comprised of one or more transistors . Each of the transistors within the series circuit, except the end transistors of the series, has the emitter coupled to the base and coUector of an adjacent one of the transistors in the series. One end transistor is coupled to the input of the amplifier and the other one of the two end transistors is coupled to the output of the amplifier. Line Powering for Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled Communication Line Interface The invention is a circuit for providing at least one dual voltage supply on an output. The circuit comprises a two-wire communication/power supply line having a positive and negative voltage on the two-wire line. A floating ground circuit provides a floating reference center voltage for the voltage supply to regulate power supphed on the line and isolate the line from ground loop noise. The floating ground circuit comprises a fixed current source circuit for providing a constant current source having its input coupled to the line and having an output coupled to the output of the supply voltage. An adjustable current source circuit is coupled to the line and to the output of the supply voltage. The output of the adjustable current source circuit is slowly varied to balance the fixed current source circuit to approximately maintain the floating reference center voltage centered between the positive and negative voltages on the two-wire line. The adjustable current source circuit is varied at a rate which is subaudible. The fixed current source is comprised of a voltage regulator and a series output load resistor. The adjustable current source circuit is comprised of a voltage regulator and a variable resistance device. The floating ground circuit further comprises a voltage divider and a gain stag amplifier. The gain stage ampHfier has an input coupled to the voltage divider and a output coupled to the variable resistance device. The voltage divider is coupled betwee the positive and negative voltages on the two-wire line. The circuit further comprising a diode bridge. The diode bridge is couple between the two-wire line and the floating ground circuit. The diode bridge provides fixed polarity to the floating ground circuit regardless of the nature of coupling betwee the diode bridge and the line. In the iUustrated embodiment the circuit is used in combination with telephon station units and wherein the floating ground circuit suppHes current to telephon station units. The circuit is also used in combination with low powered nontelephoni communication devices in which case the floating ground circuit provides predetermined and minimum amount of bias current with a smaU constant current loa on the line and to provide linear signaHng with the communication devices through th two-wire line. When used as an adapter box, the floating ground circuit further comprises diode bridge for providing a predetermined polarity from the line and a fixed curren source and voltage divider coupled across the line. The fixed current source is couple to the voltage divider to provide the floating reference center voltage and to forwar bias the diode bridge to maintain operation of the diode bridge in a linear region. One-Chip System The invention is also a telephone communication system for communicatio between a pluraUty of exterior telephone lines and a single common in house two-wir line. The system comprises a control unit having a chip for coupling to the pluraUty o exterior telephone lines. The chip controls communication between the plurahty o exterior telephone lines and the common two-wire line. A plurahty of station/adapte units each have the chip. Each station/adapter unit is coupled remotely throughout th building to the common two-wire line. The chip in each of the station/adapter unit selectively communicates with an arbitrarily selected one of the exterior telephone line under the control of the chip in the control unit. The chip in the control unit an station/adapter unit chips are identical, but operated in different modes according t their respective operational position within the system so that a one-chip system i provided and so that communication on the single common two-wire line is effecte without requiring complex control hardware within the control unit or station/adapte units. The chip in the control unit communicates with the plurahty of station/adapte units in a time frame subdivided into a plurahty of time slots. A specified portion o each time slot is reserved for communication between the control unit and each one of the plurahty of station/adapter units. AU commumcation between each one of the station/adapter units and control unit occurs within the selected portion of the time slot within each time frame. The system further comprises voice band and digital band interface circuit in the chip for communication between the control unit and the remote station/adapter units. The system is used in combination with at least one computer having modem commumcation capability and/or at least one telefax device. The chip is programmable, is capable of communicating signals in telefax compatible format and in telephone tone signal format, and is accessible within the system by extension dialing. The control unit is programmed to selectively couple the computer and/or telefax device coupled to the chip in the station/adapter units to at least one of the plurahty of exterior telephone lines through the single common in house two-wire line. The system is particularly adapted for use with a plurahty of the computers and/or telefax devices coupled to the station/adapter units. The control unit selectively couples the plurahty of the computers and/or telefax devices into a programmable network. The station/adaptor unit emulates a telephone receiver with respect to communication characteristics on the plurahty of exterior telephone lines. The station/adaptor unit further comprise a circuit for selectively powering the computer and/or telefax device upon is are accessed. The station/adaptor unit also further comprising a modem and a serial input/output port for external communication. The system further comprises an auxihary commumcation device, which is defined to include, but is not limited to a telefax machine, an answering machine, a computer or any other communication or information processing device. The auxUiary communication device has the chip for communication with the system on the single common in house two-wire line. The invention may be better visualized by now turning to the foUowing drawings wherein like elements are referenced by like numerals. Brief Description of the Drawings Figure 1 is a block diagram of the multiband audio network (MAN) system. Figure 2a is a simplified block diagram of the MAN chip used in the control unit, station unit and adapter box of the system shown in Figure 1. Figures 2b-i are block diagrams Ulustrating the external configuration of the digital section of the chip of Figure 2a for the different modes of the chip. Figure 3 is a diagram of a power spectrum as a function of frequency of th multiplexed signals on the communication line shown in Figure 1 Ulustrating a 3 kH voice channel base band, a 6 kHz digital channel, three 6 kHz AM modulated voic channels, and a 42 kHz reference signal. Figure 4 is a simplified block diagram of a portion of the digital portion of th MAN chip of Figure 2a Ulustrating the time-slot processor of the invention. Figure 5 is a timing diagram of the operation of the processor of Figure 4. Figure 6 is a time map of a time frame of communication transmitted an received by a control unit. Figure 7 is a schematic diagram of an improved crystal voltage controUe oscUlator used in the circuit of Figure 2. Figure 8 is a simplified schematic showing the circuit of the invention for drivin a bank of LEDs in the telephone station units. Figure 9 is a schematic of an iUustrated example of one implementation of th circuit of Figure 8. Figure 10 is a schematic of an embodiment simUar to that of Figure 9 wherein i has been designed to operate as a 50% duty cycle. Figure 11 is a timing diagram for the control signals used in Figure 10. Figure 12 is a block diagram of a portion of the register wherein th automatically updated transmit advance time is stored. Figure 13 is a schematic of a timing circuit wherein the stored advanced timing i used to generated a delayed timing signal. Figure 14 is a schematic of a single line communication system of the inventio wherein echo back signals are adaptively reduced or canceled. Figure 15 is a schematic of the balance network of Figure 14. Figure 16 is a schematic of the summing circuit of Figure 14. Figure 17 is a schematic of the line interface unit of Figure 14. Figure 18 is a schematic diagram of the line interface unit of the invention. Figure 19 is a block diagram wherein the concept of the line interface unit o Figure 18 is extended to multiple sources. Figure 20 is a schematic of further improvements to the DC and AC line loadin and powering of the communication line to which the interface units of Figures 18 an 19 are coupled. Figure 21 is a schematic of a prior art telephone line interface. Figure 22 is a schematic of a telephone line interface according to the invention. Figure 23 is a schematic of the balance network used in the circuit of Figure 22. Figure 24 is a schematic of the improved piezo driver of the invention used in th circuit of Figure 1. Figure 25 is a schematic of a voltage limiter of the invention. Figure 26 is a voltage gain graph comparing the performance of the circuit of Figure 25 to a prior art diode limiter. Figure 27 is a schematic of a station unit line interface according to the invention. Figure 28 is a schematic of an adaptor box (non-line powered) Hne interface according to the invention. Figure 29 is a simplified block diagram Ulustrating some of the system appHcations made possible by the invention. The invention now having been iUustrated in the above figures, turn to the foUowing detaUed description of the preferred embodiments. Detailed Description of the Preferred Embodiments The System The invention is a telephone line communication system capable of operating on a single internal telephone line as commonly found in the home or small businesses, which is economicaUy implemented with an integrated circuit chip to provide a virtuaUy one chip telephone with business system features and which provides most of the components necessary to implement a full-featured, rehable and easy to install office commumcation system. In the iUustrated embodiment, the system accepts from one to three lines from the telephone exchange with up to 14 extensions coupled to the single internal line. The extensions can include a combination of up to 8 line powered telephones (called station units) along with adapter boxes for coupling to standard telephones, modems, answering machines and fax machines. The multiband audio network (MAN) of the invention can distribute four audio bands and a digital data band over a single twisted pair of wire. The system can be routed using standard 22, 24 or 26 gauge 2-wire cable in any configuration desired by the customer without requiring separate wires to each phone such as occurs in prior art star wired systems or special multiwire cabling in other types of systems. The four audio bands on the twisted pair are used to communicate between speciaUy equipped stations or through a speciaUy equipped interface to the telephone company as described below. A 6,000 bits per second fuU duplex digital data channel is also provided on the same twisted pair for the purpose of control signaling or data communications. Turn now to Figure 1 which is a block diagram of the MAN system. A control unit, generaUy denoted by reference numeral 10, communicates with one to three outside telephone lines 12 connected to the telephone company central office through a corresponding plurality of line interface units 14. Control unit 10 includes a microprocessor 16 which dictates the switching of the four voice channels on the network to telephone Unes 12 as weU as to each station unit 18 or 36 coupled to contro unit 10 through the single multiplex twisted pair wire 20, the internal house telephon line. The operation of master chip 22, and slave chip 24, as weU as station chip 26 ar insensitive to wiring topology, that is, the routing of single multiplexed twisted pair 2 can be configured in any topology desired such as star, chained or any combinatio thereof. As wiU become evident from the description below, the system of the inventio eliminates the need for multiple, dedicated, point-to-point wiring, minimizes the numbe of components needed, and simplifies the instaUation of a telephone system. A control unit master chip 22 modulates/demodulates one of three voic channels and a digital data channel onto line 20. In addition to the three modulate voice channels, a base band voice channel is accessible aUowing up to four independen voice communications over single line 20. Master MAN chip 22 formats and modulate the data bus flow to aUow microprocessor control and monitoring of the contro signaling data of aU stations units 18 and adapter boxes 36 from control unit 10. Maste MAN chip 22 modulates the voice band signals to and from either the telephon company line or possibly a phone station onto one of four possible frequency band which aUow four communication paths to coexist on single wire pair 20. Master chip 2 controls aU telephone line functions and switching including seven direct inputs and tw outputs. It provides sound generation circuitry such as ringing/busy, signal/touch tones for the telephone line interface. The master chip 22 also includes a microprocesso interface bus which provides for its control and signaling from the central contro processor. By virtue of being the master MAN chip of the system, it has a data pat from and to this bus to and form the digital channel on line 20. The 42 kHz referenc signal provided from the master MAN chip timing provides the timing reference signa to the line 20 for the purpose of synchronization of aU MAN devices in the system Master MAN chip 22 is depicted in a more detaUed block diagram in Figure 2a and wi be further described below. Control unit slave chip 24 provides the identical functions as master chip 22 wit the exception that a separate telephone line 12 coupled to control unit slave chip 24 i serviced. In the iUustrated embodiment control unit slave chip 24 is identical i construction to master chip 22 although it is configured differently through describe below. Each station unit 18 or 36 includes aU the control and interface support necessar to perform conventional multifeatured telephone functions. These functions include combination of keyboard and display support circuitry including key strobe an debounce circuitry, LED buffers, piezo drivers, control registers and communicatio hardware. Each station unit 18 or 36 also includes MAN chip in the station mode 2 and a plurahty of telephone hardware such as a handset 28, keypad 30, LED display 32, and hookswitch 34 needed to complete a feature telephone. A MAN chip in the station mode 26 is also included within an adapter box 36 whose output is to a telephone interface 38. The telephone interface which is constructed to imitate the features of the telephone company exchange connection in turn can be coupled to any type of standard telephone line compatible instrument such as a fax machine 40, an answering machine, a standard telephone, or a computer driven modem. Station chip 26 is also identical to master chip 22 except that it is differently configured through hardware switches. Therefore, as depicted in Figure 1, one chip design is used to control and interface each telephone station 18 or adapter box 36 to each exterior telephone company line interface 14 and to control the system network over line 20. AU signals between chips 22, 24 and 26 are multiplexed over the single twisted pair wire 20 which is typicaUy up to 800 feet in length. Control unit 10 by virtue of its 3 MAN chips (1 master, 2 slaves) can transmit and receive voice on three of four bands simultaneously. Each station unit 18 or 36 transmits and receives voice simultaneously on one selected band at a time. A station unit 18 or 36 exchanges voice signaling with control unit 10 or any other station unit 18 or 36 in the case of a COM line caU or a multiple of them in the case of a conference caU. The voice band can also carry modem and fax data. The digital band is used for control of the network as weU as providing medium speed digital communication. A full duplex digital channel is provided for this purpose at a carrier frequency of 12 kHz. FuU duplex or bidirectional digital commumcation is implemented by sending and receiving data in phase quadrature, i.e. 90 degrees phase shifted signals ( A and B digital data). A power spectrum diagram as a function of frequency of the multiplexed signals on line 20 is depicted in the graph of Figure 3 which Ulustrates a 3 kHz voice channel base band, a 6 kHz digital channel, three 6 kHz AM modulated voice channels, and a 42 kHz reference signal. In the iUustrated embodiment, the three modulated voice channels operate on a center frequency of 24, 36 and 48 kHz. The base band voice channel is centered at 0 kUohertz. The two phase quadrature 6,000 bit per second digital channels are centered at 12 kHz. The 42 kHz reference signal is used by chips 22, 24 and 26 for phase-lock- loop operation in order to synchronize together. The digital data channels centered at 12 kHz are broken in to time slots which are used to control different aspects of the system. By using a scheme of dedicated time slots for the communication of signaling data as described below, 2,000 bits per second of each 6,000-bit-per-second data stream is not used even with 14 stations connected to the system. The excess asynchronous digital data stream could be used for other auxUiary services such as LCD display or to create low-speed asynchronous digital paths for smaU file transfers between computers, printers or other devices coupled to th internal telephone network. The invention treats the twisted pair bus as an input/output bus with th distinction that the remote station units are treated and addressed as input/output por to the control unit rather than as a plurahty of remote processing stations. This gener approach simplifies hardware, adds system flexibUity and improves communicatio efficiency. The system in the iUustrated embodiment is characterized as a 1-chi telephone system, this terminology denoting a system in which several copies of a singl identical chip is used in several different modes in control unit as weU as in the pluraUt of stations to implement most of the electronics of a compHcated system. As will b described below, digital and voice band filters and associated frequency modulator an demodulators in combination with precision phase lock circuits, VCO, level detector with appropriate logic, timing and control results in a telephone system not only capabl of performing control unit, station, and AB digital functions, but is flexible enough t couple to a plurahty of other compatible components resulting in extremely integrate rehable and low cost personal communication systems. Chips 22, 24 and 26 can be thought of as an input/output port-like devices, chip 26 being remote to the controlling processor, coupled by a 2-wire bus that transports plurality of analog voice grade channels, and a digital data channel and power. Th iUustrated embodiment is described in the context of that telecommunication bus using key telephone system. The reference signal at 42 kHz is transmitted from control unit 10 at all times t aUow station units 18 and adapter boxes 36 to synchronize with control unit 10 and wit each other by phase locking. Power is provided to station units 18 or 36 and are transmitted on line 20. Th power current is kept constant so that Httle or no noise generation occurs to disturb th voice and digital channels. The MAN Chip The MAN chip, whether used as a control unit master chip 22, CU slave chip 2 or station chip 26, is the same chip operated in different modes summarized in Table below. Each mode corresponds to an external switch setting, read into a particul (MODE) register in the chip at power on reset time, the output states of this registe being used to configure the chip in hardware. The modes differ depending on whethe the chip is used as a master control chip 22, slave control chip 24, or station chip 18 or 3 with their various components such as external or piezo ringer or with maximum minimum input/output configurations. Table 1 Mode Function Figure 000 Control unit master 22. Controls the digital 2b channel. SuppHes timing reference for the MAN line. Connects to one telephone company line. 001 Control unit slave 24. Only receives digital 2c channel for PLL synchronization. Connects to one telephone company line. 010 Station unit 18 or 36. Station mode with 2d microprocessor interface and external ring circuit. Oil Station unit 18 or 36. Station mode with 2e microprocessor interface and piezo ringer 100 Station unit 18 or 36. Station mode with maximum 2f input/output configuration and external ring circuit. 101 Station unit 18 or 36. Station mode with maximum 2g input/output configuration and piezo ringer 110 Station unit 18 or 36. Station mode with minimum 2h input/output configuration and external ring circuit. 111 Station unit 18 or 36. Station mode with minimum 2i input/output configuration and piezo ringer The MAN chip 22, 24, and 26 is diagrammatically depicted in Figure 2a. The MAN chip, here collectively denoted by reference numeral 60, is comprised of an analog section 86 and digital section 84. Digital section is comprised of a register file 58 which is used for various control and input/output operations. How registers 58 are used depends upon the mode in which chip 60 is operated as depicted in Table 1 above. InternaUy, MAN chip 60 operates on an 8-bit data word. The upper 4 bits identify the address of a 4-bit register and the lower 4 bits are the data in the register. The 4 bits of address aUow 16 possible 4-bit registers to be addressed. To aUow for additional lesser used functions and ports, secondary registers are also provided which can be addressed through two successive words. The first word's upper 4 bits are 1000 or a code "8" which indicates that a secondary register is to be addressed. The lower 4 bits of the first word identify the address of the secondary register. The second word is the secondary register's data. Up to 16 secondary registers can be addressed. The registers within register file 58 when used in the control unit modes 000 or 001 are accessed through a microprocessor interface 98 on chip 60. The data signals are provided on an 8-bit bidirectional data bus 42, an address line (A0), read/write contro line (RD/WR) and interrupt request Hne (IRQ) aU included as part of control bus 56 See the control mode configurations of Figures 2b and 2c. Register file 58 when configured in a station unit mode (010 - 111) is accesse through the MAN line's digital channel by way of its data bus 42 and communication interface 88. The states of these registers are then written-to or read-from by the centra processor 16 through the control unit master chip 22 of Figure 1 through it corresponding microprocessor interface 98 via its data bus 42 and commumcation interface 88. Consider first the registers in register file 58. In the iUustrated embodiment, th definition of the registers in file 58 change depending on whether the register file is in station mode and control mode. The register assignments are defined below in Table according to whether it is acting as an input register, output register or secondary outpu registers. The register name, 0 - F, is in the first column of Table 2 while the registe function is summarized in the next two columns of Table 2 for a station mode (010 - 111) or control mode (000 or 001) respectively. Table 2 Input Registers (4 bits) Reg Station Mode Function Control Mode Function A0 W/R 0 Mode/ no change used internaUy ~ — during initialization 1 Mode/ no change used internaUy ~ after initialization 2 Inpins A not used — — 3 Inpins B not used ~ — 4 Keys A not used — — 5 Keys B not used — ~ 6 Keys C not used — — 7 Keys D not used ~ — Output Registers (4 bits) Reg Station Mode Function Control Mode Function A0 DIR 8 Secondary Reg. Select Secondary Reg. Select 1 W 9 Green Not used — - A Red A Not used — - B Red B Not used — - C Serial Control PL Control 1 w D Channel Control Channel Control 1 w E DTMF DTMF 1 w F Sound Sound 1 w - Not used Status Register 1 R - Not used Communication Reg. 0 W - Not used Communication Reg 0 R W = write; R = read; A0 is control line input Secondary Output Registers (81 its) Reg Station Mode Function Control Mode Function A0 DIR 0 Output Exp. Reg #0 Not used — - 1 Output Exp. Reg #1 Not used — - 2 Output Exp. Reg #2 Not used — - 3 Output Exp. Reg #3 Not used — - 4 Test Mode Not used — - 5 Test Mode Not used — - 6 Slow Event Reg Not used — - 7 Not used Not used — - 8 Program Tone A Program Tone A 1 w 9 Program Tone B Program Tone B 1 w A Effects for Sound Effects for Sound 1 w B On/Off Time On/Off Time 1 w C Timing/Balance Timing/Balance Control 1 w Control D Gain Control Gain Control 1 w E Not used Not used — - F Not used Not used — - W = write; R = read; A0 is control line input Sound section 96 on chip 60 generates sounds that are commonly used i telephone equipment. Sound section 96 is active in aU the modes for chip 60 and can b configured either to operate a high voltage ringer or piezo ringer. Sound section 9 generates a tone A and tone B. A third section within sound section 96 is an On or O timer which controls the On and Off duration of tone A and tone B. A warble functio can be provided with switches between tone A and B at a low frequency rate to create ring sound commonly used in telephones and telephone systems. Sound section 96 ca either be used in a fixed mode or can be programmable. The fixed mode typicaU includes aU the DTMF tones, dial tone, two busy tones, and two ring tones. Fixed tones are typicaUy set by a one mask ROM within chip 60. In the programmable mode, tones and On/Off times can be altered by writing into appropriate registers under program control. Shown in Figure 2a is both an internal mode associated with TLA and TIB and external mode associated with TEA and TEB for tone A and tone B. In some modes, tone A and tone B are added internaUy within sound section 96 and drive a piezo ringer with TEA and TEB. Since the signals TEA and TEB are made the complements o each other in this mode, this effectively doubles the voltage across the two pins for use with a piezo ringer. In other modes, a high voltage ringer is used instead of a piezo ringer and TEA is used to output a 32 Hz square wave with TEB the ringer enable and are used in conjunction with the LS pin used for High Voltage Sense. The On/Off timer within sound section 96 controls the ringer enable On/Off times of the ring signal. Both internal and external sounds can be fixed or programmed. In the internal mode, tone and tone B output to separate I/O pads as TLA and TIB and these are used for the DTMF tones, dial tones and busy tone. Any adding of the TLA and TIB signals must occur outside of chip 60. Chip 60 includes a serial input/output port 98 which aUows for a simple interface to a standard 8-bit microprocessor 16. See Figures 2b-e. Input/Output section 98 also aUows for detection of switch closure using a multiplexing approach. Up to 10 strobed lines can be strobed by MSTB, ISTB and ESTB using external hardware of whic strobed lines, eight key input lines 21 are coupled through an external shift register 1 and two are status switch lines 23 are provided for a maximum of 79 switch locations 19. See Figures 2d-g. MSTB and EIPA 0-3 provide the mode and MSTB and EIPB 0- provide the station identification number for chip 60. Strobe signals on these pins ar normaUy high and go active when low. The strobe line puUs the EIP line low through closed switch that is connected to the strobe and EIP line. Otherwise, the EIP lin remains high. Key switches are debounced internaUy within chip 60. Mode and statio identification switches are not debounced. The status of a key state is transmitted afte being in a new state for approximately 24 milHseconds. The momentary key status is transmitted immediately after it is pushed down or closed and is not considered released or open untU it has been in that state for at least 24 milHseconds. There are four banks of status switches which are multiplexed on the EIPA and EIPB pins besides the keyboard switches in the station unit modes. Each bank has four switches for a possible total of 16 switches. The banks are caUed mode, station, Inpins A, and Inpins B switches. Both mode and station identification number use the MSTB pin shown in Figure 2a as the strobe. The switches Inpins A and Inpins B use the pin ISTB for the strobe. PKSO-3 are used as the strobe signals for momentary key switches when chip 60 is configured in a minimum input/output configuration and where 32 key switches are supported. See Figure 2h and i. In other input/output configurations where a maximum number of 63 key switches are supported, an external shift bit register is connected to the strobe signal of MSTB whUe the clock input of the external shift register is connected to ESTB. The outputs of the shift register are then used as the strobe lines for a key switch matrix for key denoted as keys A keys C and keys B and keys D, each group being a 4 x 4 submatrix. Figures 2d-i Ulustrate some of the display and communication capabiHties of chip 60 when used as a station unit 18 or 36. Each of the Figures 2d-i show coupling of the serial data output pins, SDO and SDCK to load a shift register 25 whose output in turn is coupled to a LCD display 27. In the modes shown in Figures 2d and 2e microprocessor 16 has its serial data pins coupled to an RS232 level converter 29 which is coupled in turn to serial connector 31 to auxUiary data equipment such as modems, faxes and the like. When in the modes depicted in Figures 2g and 2f shift register 25 is also coupled to four secondary registers 33 clocked by PKSO-3 from chip 60 to provide additional expanded outputs. The modes of Figures 2f-i further iUustrate control of a plurality of green and red LEDs 35 provided on the station unit by pins P0G3-P7G1 of chip 60 which operate to provide telephone Hght status functions. Considering now the analog section of the circuit. Turning first to the voice channel, each MAN chip 60 includes an analog modulator 62, an analog demodulator 64, a data modulator 66 and a data demodulator 68. Analog modulator 62 takes an audio signal to be transmitted, T which is buffered by buffer 70 and filtered with a 3 kHz low pass filter 72. The analog signal is then modulated by analog modulator 62 to a higher frequency channel using conventional AM suppressed carrier modulation techniques or passed through in the case of base band. As previously stated, three voice carrier frequencies are avaUable, namely 24, 36, and 48 kHz as deHvered to modulator 62 from clock generator 82. The carrier frequency is determined by Register D. Clock generator 82 in turn is driven by oscillator 100 and controUed by control circuit 59. When the VCO osciUator is used in an oscillating mode, a crystal and a resistor capacitor network is tied externaUy from VCXI and VCXO. The VCO osciUator is buffered with 102 and appears on output pin VCXB for driving other circuitry. If the VCO osciUator is driven from another source, the signal is entered at pin VCXI. The modulated signal from 62 is then appHed to output TXO through a summing buffer 74. TXO output is further conditioned outside the MAN chip and then appHed to MAN line 20 (Figure 1). In addition to these three modulated voice channels, the analog signal TA can also be appHed to MAN line 20 as a base band signal, i.e. a signal without a carrier frequency. Analog demodulator 64 receives an input signal, RXFP from MAN line 20 (after some conditioning) through amplifier 76. Modulator circuit 64 demodulates the received signal back to the base band from the appropriate carrier frequency, if any, filters the output with a 3 kHz low pass filter 78, and then couples the signal through buffer 80 to generate a received audio signal, RA Analog modulator 62 and demodulator 64 are synchronized to the signals on MAN line 20 (Figure 1) by means of a clock generator 82 and (depending on the mode) by a phase lock circuit. Master control unit 22 (Figure 1) transmits a 42 kHz sinusoid on MAN line 20 (Figure 1) as depicted in Figure 3. This is used to synchronize the analog and digital channels between control unit 10 and station units 18 or 36. Station units 18 or 36 as weU as the slave control chips 24 phase lock onto the same 42 kHz reference signal. Phase locking aU chips to the 42 kHz signal on the MAN line 20 helps to compensate for any phase shift that may occur on MAN Hne 20 or the interface circuitry to that line. As depicted in Figure 2a, the phase lock loop is implemented by receipt of the 42 kHz reference signal through a sample and hold circuit 90 which is coupled to MAN line 20. The reference signal is demodulated by demodulator 92 and output to PLLO and conditioned externaUy and fed back into the PLIN pin for digital phase lock is required and into pin VON when analog phase lock is required. PLIN is conditioned and input to clock generator 82 in order to address the clock timing on chip 60 to synchronize it with the reference signal in a digital mode VCO implementation (with the analog VCO fixed to a set frequency). VCIN is buffered and input to the analog VCO osciUator 100 for an analog mode VCO implementation. The choice of digital phase lock loop or analog phase lock loop is selected with one bit of the mode register which is loaded at power on from an external pin. In the mode where chip 60 is used as master chip 22 (Figure 1) synchronization is made directly to the on-chip 42 kHz reference signal. The operation of the data channel is similar to the analog channel except that only one data channel is avaUable. Digital data from data bus 42 is coupled to communications interface 88 whose output is coupled to a 3 kHz low pass filter 91 and then to modulator 66 where the digital data is modulated with a 12 kHz carrier frequency. The output of modulator 66 is coupled to summing buffer 74 and then through pin TXO to MAN line 20 after some conditioning. The use of amplitude modulation aUows the signal 12 kHz data carrier frequency to have independent receive and transmit signals, transmitted in phase quadrature. Therefore each transmitted bit has three states, logical 1 ( + V), logical 0 (-V) and NO ENERGY (V = 0). The carrier suppressed digital signal is received from MAN line 20 as RXFP and conditioned by amplifier 76. The received data signal is demodulated by demodulator 68 whose output is then coupled through a 3 kHz low pass filter 92. The output of filter 92 is then coupled to the input of digital interface 88. The data is then transferred to the data bus 42 where it is transferred into registers. The combination of a modulator and demodulator in chip 60 is to give chip 60 fuU duplex 6 kUobit per second capabUity. Digital interface 88 is also coupled to control bus 56 and has as it outputs the clocking signal serial data output, SDO, a 6 kHz bits per second serial data out clock, SDCK, and a serial data latch signal, SDL. Therefore direct serial communication is provided to other serial devices from the MAN line 20 (Figure 1). A Simplified Bus Oriented Processor Using a Fixed Time Slot Protocol Current state of the art processors include logic decoders that convert a program counter into an instruction pointer that references an instruction stored in a read-only memory and implement jumps between instructions. The invention incorporates a digital section or processor 84 shown in the block diagram of Figure 2a in each chip 22, 24 or 26 of Figure 1 that has a fixed cycle with no jump capabUity. The program counter of the prior art is replaced with a timing or state counter which eliminates the program stored in a read-only memory in favor of local control signals derived from the timing. The processor uses a πώiimized arithmetic logic unit which is described in the present specification as a universal logic unit (ULU) to make increments, decrements and comparisons instead of additions and subtractions. The universal logic unit (ULU) is defined in the specification as logic circuitry that operates each cycle to perform universaUy used logic functions simUar in nature to the role of an arithmetic logic unit in a larger conventional system, but in a manner consistent with the simpler orderly protocol described below. As a result, less sUicon area is required in the integrated circuit constructed to accomphsh these tasks whUe maintaining similar programmabUity and functional equivalency to a conventional processor on a low level. Turn to Figure 4 which is a block diagram of circuitry operating according to the cycUcal timing iUustrated in the timing diagram of Figure 5. Digital section 84 of Figure 2a includes a data bus 42, a universal logic unit 44, registers 46 comprising register file 58, an RS flip/flop 48 in combination with logic gate circuitry 50, 52 and 54 as wiU be further described below comprising part of control circuit 59 of Figure 2a. Logic gate 50, 52 and 54 in turn are coupled to a timing/control bus 56. Data bus 42, which is 8 bits wide in the iUustrated embodiment, is a precharge bus driven by universal logic unit 44 and registers 46. Bus conflicts are resolved b assigning each operation to a dedicated time slot. The time slots are diagrammaticaU depicted in the timing diagram of Figure 5. Each time slot in turn is comprised of three cycles. The use of precharged data bus 42 aUows low power, minimum hardware and low noise performance in the system. Data bus 42 is precharged to high voltage representing a logical 1, but is inverted on being coupled to any register so that the precharge state is seen by the circuitry in Figure 4 as a logical 0. This aUows registers 46 to be easUy cleared by inhibiting aU read cycles during one time slot and to write each register to be cleared in its individual time slot. Universal logic unit 44 operates on data presented on data bus 42 in conjunction with registers 46 to do compares, increments and decrements. ULU 44 may operate on data in every time slot. ULU 44 in the iUustrated embodiment is constituted to perform only four instructions represented by the 2-bit instmction codes set forth below in Table 12. Table 12 Instmction Definition COMP INCR COMPI compare and increment, 1 1 read to bus on NO COMPARE CMPD compare and decrement 1 0 read to bus on NO COMPARE INC increment 0 1 read to bus on NO CARRY DEC decrement 0 0 read to bus on NO CARRY The most significant bit of the instmction is a signal referenced in Figure 4 as compare, COMP, and the least significant bit is a signal referenced as increment, INCR. Th control signals COMP and INCR are provided to ULU 44 on timing and control bus 5 through logic circuit 50. Each time slot includes three cycles and the operation of eac cycle for the four instructions is summarized and described in tabular form in Table 13 which defines the operation of ULU 44 in response to COMP and INCR. Table 13 Instr Cycle CMPCRY X'fer bus to ULU X'fer ULU to bus CMPI 1 N Y N & CMPD 2 N Nl N 3 Y2 N X3 INC 1 N Y N & DEC 2 Y N N 3 γ4 N X N = no Y = yes X = optional 1 - The bus is not transferred into the ULU, but the ULU compares the data stored in cycle 1 to the data on the bus in cycle 2. 2 - CMPCRY is set to a "1" if the values compare and set to a 0 otherwise. 3 - If CMPCRY is a "1", the incremented or decremented value is not driven onto the bus in cycle 3, otherwise it is. 4 - If the incremented or decremented value produces a carry, CMPCRY is set to "1", otherwise it is set to a "0". The signal compare/carry, CMPCRY, is an output signal from ULU 44 and is used by other logic circuitry in Figure 4 to determine the subsequent operation to be performed based on whether a compare or carry has been detected. The first cycle of each instmction writes data from data bus 42 to ULU 44. For the instructions CMPI and CMPD, data is written to ULU 44 in cycle 1 and compared to data on the data bus in cycle 2. For CMPI and CMPD, if the data on data bus 42 in cycle 1 and cycle 2 are equal, that is a vahd comparison is made, CMPCRY is set to "1" in cycle 3 and no data is written from ULU 44 then on to data bus 42. For CMPI, if the data on data bus 42 in cycle 1 and cycle 2 do not compare, i.e are not equal, CMPCRY is set to "0" in cycle 3. ULU 44 would then increment that data it received from data bus 42 in cycle 1 and drive the incremented value on to data bus 42 in cycle 3. The same operation occurs with respect to CMPD except the value is decremented. In the instructions INC and DEC, the data on data bus 42 is ignored in cycle 2. For INC, if the data bus is aU l's in subcycle 1, CMPCRY is set to "1" in cycle 3 and ULU 44 does not write onto or drive data bus 42. For INC, if the data in cycle 1 is not aU l's, the data on data bus 42 in cycle 1 is incremented and ULU 44 writes or drives the result onto the data bus in cycle 3. In this case, CMPCRY is set to "0" in cycle 3. For INC, if nothing else drives data bus 42 in cycle 3 for a carry condition, i.e. CMPCRY equal to "V, then incremented value "00" is written onto the bus by default. For the instmction DEC, if the data in cycle 1 is not aU 0's, the data on data bus 42 in cycle 1 is decremented. ULU 44 drives the data bus 42 in cycle 3 and sets CMPCRY to "0" in cycle 3. For DEC, if data bus 42 is aU 0's in cycle 1, CMPCRY wiU be set to "1" in cycle 3 and ULU 44 wUl not write onto or drive bus 42. For the instructions CMPI and CMPD, the control signal CMPCRY is vaHd only during cycle 3. For the instructions INC and DEC, control signal CMPCRY is valid in both cycles 2 and 3. These basic instructions can be used to build up aU of the more complex operations necessary to operate digital section 84 of Figure 2a. Registers 46 may be any type of register such as simple transparent latches that read data from data bus 42 and store it indefinitely. Registers 46 also write data to data bus 42 by leaving bus 42 precharged or driving it to ground. Register-to-register transfers are accomplished by having one register 46 configured to be in a read cycle at the same time as another register 46 is configured to be in a write cycle. The read and write control signals are determined by decoding the timing bus 56 for the time slot information and from control signal CMPCRY or other logic signals for additional information. The number of registers 46 which may be coupled to data bus 42 is limited only by the capacitance of data bus 42. RS flip/flop 48 is used to interpret the control signal CMPCRY along with other control signals from timing bus 56 and other conditional logic signals. Flip/flop 48 stores control or logic information for subsequent use. The output of flip/flop 48 is used as an additional condition logic term or control signal. Although only one flip/flop 48 is depicted in Figure 4, it is expressly contemplated that a plurahty of such flip/flops wiU be included as needed. A simple counter can be mechanized by reading a value from a register 46 in Figure 4 when CMPCRY is set to a "1" and incrementing it with ULU 44 at the time or event occunence that needs to be counted. A simple comparator can be mechanized by reading one source to data bus 42 during cycle 1 and another source to data bus 42 in cycle 2. CMPCRY from ULU 44 is then observed in cycle 3. By using flip/flops and other conditional logic elements more compHcated counters, comparators and data transfer circuits can be implemented. It must be understood that it is within the scope of the invention to provide an even simpler processor by eliminating ULU 44 and replacing it with custom logic circuits for performing simpler logic functions in more basic appHcations or to create a more complex processor by replacing ULU 44 with a more extended arithmetic logic unit. The operabiUty of processor of Figure 4 may also be extended to port data bus 42 directly to input/output pins for operation of the circuit or to port data bus 42 to other existing buses such as an external microprocessor data bus. Thus, it can be readily appreciated that by combining a simple ULU 44 with simple registers 46, RS flip/flops 48, data bus 42 and timing bus 46, many operations involving timers, counters, comparators, and data transfer operations can be performed with a minimum number of interconnections and logic elements with the result that only a smaU portion of the integrated circuit is used to implement these functions. Utilizing a precharged data bus 42 with dynamic memory unit such as registers 46 and 48 also results in a smaU usage of integrated circuit area, low power, and low noise whUe aUowing the implementation of multiple timing and comparison functions. By performing the instructions in a time slot scheme based upon timing signals from timing bus 56, multiple timing, comparison and transfer operations can be easUy realized. The timing signals required for such a time slot scheme are generaUy avaUable on timing bus 56 without the addition of extra hardware on the chip. The programming avaUable within the time slot scheme can be expanded by increasing the frame time period. As a result of the time slot bus scheme, the hardware required to perform timing, comparison and transfer operation is minimized in terms of the die area required for its implementation and wiU usuaUy be able to take advantage of counter devices needed for other system functions. A much simpler solution for tasks performed by a processor of Figure 4 results than would be realized if instead instmction execution were performed with a program counter with jump capabUity. The combination and smaU number of instructions necessary, in combination with the time bus slot scheme, yields a simple easy-to- implement instmction decode within ULU 44. Communication Signaling Protocol A synchronization signal is transmitted for phase locking aU the station units (station phone 18 or adapter box 36 of Figure 1) to the control unit clock and a common frame synchronization signal is used to align aU units in time. Each remote telephone or adapter box has a unique number assigned to it by the setting of a manual switch on that phone or box. Each number (manuaUy set) corresponds to a slot in a time frame as referenced from the frame synchronization signal. Thereafter aU control signaling to a specific station unit occurs in that particular time slot. This eliminates the need in the system to estabhsh a handshake protocol every time a message is sent in either direction. Slow downs in system response to stimuli during periods active commumcation between a plurahty of units is thereby avoided. Serial digital data can also be transmitted over the same line without affecting signaling speed. Instruments such as a Hquid crystal display can be serviced quickly withou affecting signaHng response time and a good enor correction scheme can b implemented also without affecting signal response time. Other serial data can be quic transferred for use as data communication network regardless of the direction o transfer between the units as controUed by the master control unit. Chip 60 in Figure 2a in the station mode 18 or 36 in Figure 1 also perform digital commumcation protocol functions to faciUtate proper data communication between it and chip 60 in the control unit mode 10 of Figure 1. Communicatio protocol is used to define how data on the digital channel is sent. The communicatio protocol of control unit 10 is implemented by a microprocessor 16 which is externaU connected to device 22. Turn now to Figure 6 wherein a timing frame is diagrammaticaUy laid out in Hn 104 as a timing frame transmitted by the control unit 10 in Figure 1 and a timing fram 106 for signals received by control unit 10. In the iUustrated embodiment, timing frame 104 and 106 are each divided into 16 time slots and each time slot is divided into 3 bytes Time progresses from left to right in the diagram. A sync signal of "FFF5" is transmitte from control unit 10 once per frame to synchronize station units 18 or 36 in Figure 1 t control unit 10. Fifteen of the time slots bytes, ST0-ST14, are used for the statio control signaling data. The control signaling data is data transferred from control uni 10 to a station unit 18 or 36, or visa versa, that relates instmction or status informatio such as key presses, off-hook switch status, ring, select channel and the like. Input dat is considered data sent from an input device such as a keypad or off-hook switch fro station unit 18 or 36 to control unit 10. Output data is defined as data used to contro the output devices such as the ringer or LED displays sent from control unit 10 t stations 18 or 36. The time frame organization as depicted in Figure 6 is used to regulate the dat flow of the 6 kilobit per second serial data stream on MAN line 20 in Figure 1. Bar 10 is a frame for a transmission from control unit 10 and bar 106 shows a frame fo reception to a control unit. Three consecutive bytes create a time slot. Sixteen tim slots create a frame. Each of the 14 station units is uniquely assigned a dedicated tim slot in the frame during which it communicates with control unit 10 or more specificaUy master chip 22. Fifteen of the time slots bytes are dedicated to corresponding statio units, STO-14. The first byte of a time slot is reserved for serial data channel A , th second byte is reserved for serial data channel B , and the last byte is used for contro signaHng data to communicate with one of the station units 18 or 36. Serial dat channels (A or B) can be opened up between the control unit and any one of the statio units as a high speed data link. One time slot in each frame is reserved for the frame/byte sync character. The frame organization aUows master chip 22 to communicate with the 15 station units in a round-robin fashion. The frame/bit synchronization slot's first byte is reserved for serial A data. The second and third bytes contain the data "FFF5". This frame/byte sync word sets and maintains the byte's synchronization, that is a grouping of 16 bits to form a serial data stream to synchronize the station units and control unit. Only master control chip 22 transmits in the control unit transmission (bar 104). Master chip 22 sends and receives its control information as weU as aU of the data transmitted and received on the digital control channel (after proper conditioning, described elsewhere) directly to and from microprocessor 16 on microprocessor data bus 23. The most significant bit of each byte is always sent first. The transmit time slot byte of a particular station unit is delayed 5 bytes from its receive time slot byte. This aUows the station unit 18 or 36 to respond to a command received from control unit 10 in the same frame, so that the station unit can send an echo back of the command received from control unit 10 in the eariiest avaUable byte time. When chip 60 is operated in a station unit, it has three communication modes that determine the state of its digital communication link with master control unit 10. The three states are bad frame (BF), not bad frame (NBF), and good communication (GC). A power-on-reset signal (POR) from circuit 103 in Figure 2a in station unit 18 or 36 forces the chip into a bad frame mode when the chip is first powered up. The chip goes from bad frame mode to not bad frame mode when a frame/byte sync word is received. If station unit 18 or 36 receives a second frame/byte sync word in its proper time slot position, the chip wiU then go from a NBF mode to a GC mode. It wiU drop back to the NBF mode if the frame/byte sync word is not received at the proper time. Station unit 18 or 36 returns to the bad frame, BF, mode if it faUs to receive the expected frame/byte sync word for three consecutive frames in the proper designated time slot. Any station unit 18 or 36 if in a bad frame, BF, mode wUl inhibit transmitting any signal, initializes out all registers, reads the mode and station number from fixed input pins and periodicaUy walks the carrier phase, i.e.. phase shifts its internal clock around the 42 kHz reference signal in an attempt to achieve synchronization. In the not bad frame mode, NBF, aU transmission of signals is inhibited and the frame and byte counters are initialized. In the good communication, GC, mode, the data transmitter 74 in Figure 2a is enabled and the station unit 18 or 36 enters the normal operating mode. When MAN chip 60 is used as a slave chip 24, it operates in a manner simUar to that just described with respect to station units 18 or 36 with the exception that the slave chip 24 never transmits serial data. Instead, it sends and receives its control informatio directly to and from microprocessor 16 on microprocessor data bus 23. When chip 60 is used in the master control unit mode its operation is simU except there is no frame/byte sync detection since no frame/byte sync characters a sent by any of the station units 18 or 36, the master control chip is the defact synchronizing standard in the system, and is thereby not in need of synchronizin Anything written into communication register 88 in the master mode is transmitted the next byte time regardless of the time slot or frame count. The frame/byte sy character is not generated by the master control unit 22, but is provided microprocessor 16 by writing the bytes FF and F5 at the appropriate times consiste with the frame organization described above in Figure 6. The microprocessor's interrupt line, IRQ, 85 in Figure 2b is set low when contr unit 10 is ready to transmit and receive 8 bit data words through the digital channel. communication register 88 is not written to by processor 16 between interrupts, then n energy is transmitted for that byte time. If a vahd byte has been received by the mast control unit from a station unit,, then the most significant bit of the master chip's stat register is set and can be read by processor 16 when the interrupt Hne is set low. In th master chip mode, communication starts with a BF mode after power-on reset. Aft the mode is read, master MAN chip 22 goes to the NBF mode. One frame later, chip 6 goes from the NBF to the GC mode and stays in the GC mode thereafter. In the G mode, the master chip digital transmission on MAN line 20 is enabled. Control unit 10 transmits data to station units 18 or 36 to change outputs such the ringer or Hghts or to echo back data received from the station units for the purpos of error detection. Table 2, 3 and 4 Ulustrates the data format for these output byt along with their assignments to the registers either in control unit 10 or station unit 18 36 included within register file 58 within each of the chips. Normal data transactio are handled in single byte transfers summarized in Table 2. However, lesser use operations require two bytes of data summarized in Table 2. The format of the sing byte transfers are iUustrated in Table 3 and the two byte transfers in Table 4 below. Table 3 Single Byte Data Transfer Register number - 4 bits Value - 4 bits Table 4 Two Byte Transfer of Data the first byte 1000 (Reg #8) - 4 bits secondary reg. # - 4 bits the second byte eight bit value In the iUustrated embodiment, aU control information transfers from control unit 10 to a station unit 18 or 36 begin with a register number "8" or greater as these registers represent output terminals to the station unit. For a 2-byte data transfer, the first byte is sent in a time slot for a given station unit and the second byte is sent in the same time slot for the same station unit but in the very next time frame. Control unit 10 sends a NO ENERGY signal in a particular station's time slot if it has no information to transfer or if it is sending an error message. Control unit 10 sends a NO ENERGY by placing substantiaUy zero voltage signal on MAN line 20 as opposed to the data 1 (+ V) or 0 (- V) voltage level. Station unit 18 or 36 transmits data during the signaling byte in its assigned time slot as depicted in time frame 106 in Figure 6. If an error occurs, a NO ENERGY signal wiU be sent during its assigned time slot instead. Data transfers initiated by station unit 18 or 36 to control unit 10 are input data to the station unit and are sent on a priority basis as summarized in connection with Table 5 below. As summarized in Table 2, all transmissions in the iUustrated embodiment initiated by a station unit 18 or 36 have a register number less than "8" as these registers represent input terminals to the station unit. The debounced keyboard switch closure data has the highest priority of any data transfer initiated by station unit 18 or 36 so that keyboard data wiU not be lost from a fast sequence of key presses by a user. As summarized in Table 2, keyboard information is presented in registers #4 through 7. Each register has a 4-bit value which represents one of 16 keys. In register #6, the value 0 is reserved for "no key", that is, no key is pressed. Therefore, 63 possible key can be recognized and transfened. Data which indicates the status of discrete inputs connected to input ports A or B has the next highest priority of data which can be sent by a station unit. This data is debounced and then sent as the contents of registers #2 and #3 as summarized in Table 2. Each bit value represents an independent status state, therefore, each register represents four inputs. These registers are sent from station 18 or 36 to control unit 10 during initialization and then individuaUy every time thereafter whenever there is a change in one of the inputs connected to the ports A or B. Station unit 18 or 36 sends "key-up" data "60", that is, register #6, value 0 = "no key", after each key has been pressed and is finaUy released, unless another key press is ready to be sent in which case the last key up is ignored. This procedure enhances key debounce routing in the microprocessor as weU as sound feedback to key depressions. The mode register is sent from station unit 18 or 36 during initialization as the register #0. After initialization, the same mode register data is sent as register #1 and has the least priority as it should rarely change so that if no data is avaUable, the mode register is sent as an idle byte. Effectively, register position is being used in this case as a form of station information from station unit 18 or 36 to control unit 10. As depicted in Figure 6, the serial A and serial B data fields are each eight bits of data with no control signaling information. Control unit 10 instructs station unit 18 or 36 to receive serial data, send serial data or both. Control unit 10 wiU also instruct station units 18 or 36 to send and receive the serial A bytes only, the serial B bytes only, or both. If control unit 10 or station unit 18 or 36 receives any bit as NO ENERGY in a serial byte when it is selected, it wiU interpret the transmission as being no data. If control unit 10 or station unit 18 or 36 does not have data to send in the serial A or serial B bytes when it is selected, it wiU transmit a NO ENERGY signal. If a serial data stream (A or B) is selected, the byte in every time slot is avaUable, except for the FF byte in the frame sync time slot. To faciHtate rehable communication, the MAN system uses an acknowledge type protocol to detect data and system errors. BasicaUy, the acknowledge, ACK, is an echo of the byte received. When station unit 18 or 36 transmits a byte to control unit 10, station unit 18 or 36 expects to receive an echo of its transmitted byte (except the idle byte) back from control unit 10 in the station's next received time slot. If an echo is received, station unit 18 or 36 considers its byte as being accepted by control unit 10. If station unit 18 or 36 receives any other response from control unit 10 in the station's next received time slot, station unit 18 or 36 enters an enor condition. In the enor condition, station unit 18 or 36 wiU send a signal, NO ENERGY, in its next transmit time slot instead of new data. When control unit 10 receives the signal, NO ENERGY, from station unit 18 or 36, it knows that station unit 18 or 36 has encountered an enor. After station unit 18 or 36 transmits the signal, NO ENERGY, it wiU wait until the next avaUable time slot and then retransmit the original byte. The process wiU then be repeated until the prope echo is received. When control unit 10 transmits a byte to station unit 18 or 36, it also expects t receive an echo of its byte back from station unit 18 or 36 in that station's next receive time slot. If a correct echo is received back, control unit 10 knows that station unit 18 or 36 has acted upon control unit 10's data correctly. If the data echoed back from the station unit is inconect data or NO ENERGY, the control unit wiU retransmit the data at the next opportunity. The faUure of station unit 18 or 36 to echo back control unit 10's byte correctly can occur for several reasons. One reason is the byte was received inconectly by either the station unit or control unit. Another possibihty is that station unit 18 or 36 has something of higher priority to send. When station unit 18 or 36 has more than one byte to send control unit 10, priority is set forth in Table 5 below which is used to determine which byte is sent first. Table 5 Priority Action highest error condition - send NO ENERGY retransmit data not properly echoed echo back of secondary reg data keyboard switch closure switch change (input ports A and B) echo back of received data key up command lowest register #1 (register #0 before initialization) Note that station unit 18 or 36 always sends data to control unit 10 in its time slot except for an error condition. If station unit 18 or 36 has no new data to send, then it wUl send contents of register #1. If station unit 18 or 36 sends a signal, NO ENERGY, then station unit 18 or 36 is in an error condition. The ttansmission of the contents of register #1 do not require an echo back from control unit 10 when they are transmitted by station unit 18 or 36. Consider now the sequence of events which characterize control communication on the single two-wire pair. After successful power-up, a 42 kUohertz reference signal is transmitted on MAN line 20. As soon as the master chip in control unit 10 achieves good communication, mode GC, it begins to transmit. A frame sync is transmitted at the beginning of each frame. After the first byte of data is received by control unit 10 in a time slot dedicated to a station, control unit 10 recognizes that a station unit is on-line in that time slot. If control unit 10 receives four consecutive frames of no energy in that control unit's time slot, it wiU take the station unit in question off-line. After a station unit is on-line, control unit 10 is now ready to accept data fro and send data to that station. Consider first the reception of data by control unit 10 Each time a vahd input byte of data is received from a station that is on-line, contro unit 10 reads the data and echoes the same data byte back to the respective sendin station unit at the first opportunity. A vahd input byte of data is a byte which has no bit at the no energy or zero voltage level and the most significant bit is a logical 0. If ther is a signal or energy in the next byte received from the station unit in question, it mean the previous data byte received was good and control unit 10 may safely act upon it However, if there is a NO ENERGY bit in the next byte, then the previous data byt may be in error and the transmission is ignored. The first byte received from a station unit after it comes on-line is the input po B data. In control unit 10, this establishes the states of inputs B switches or register # for that particular station unit. After station unit 18 or 36 sends the initial inputs information, the station unit only sends changes to inputs B to control unit 10. Next, the input port A data is received from the station unit corresponding to th states of inputs A switches or register #2, and is treated in the same manner as inpu port B data described above. After the input port A data, control unit 10 is sent mode information in the for of the address and contents of register #0 which communicates the station unit's mod or configuration. Station unit 18 or 36 continues to send register #0 an idle byte unti control unit 10 resets the initialization status bit in the station unit, which wiU then caus the station unit to send an idle byte in the form of the address and the contents o register #1. Control unit 10 is now ready to accept new input data from the station unit i question as long as it is on-line. If the byte has energy in it, but is not a vahd input data i.e. the most significant bit is equal to a 1, then NO ENERGY is sent in the nex opportunity to the station unit to show an enor. Consider now how data is transmitted from control unit 10 to station unit 18 o 36. Each time control unit 10 sends a byte of data to station unit 18 or 36, control uni 10 looks at the next byte of data received from that station unit. If the next receive byte is the same data that conttol unit previously sent, then control unit 10 knows tha station unit recognized the previous transmission and acted upon it. If the next byte sen by the station unit is anything else, the possibiHty exists that data was in error and it wU be resent by control unit 10 at the next time slot opportunity for the station in question. After a station unit comes on line, control unit 10 sends the message to set th initialization status bit in the station unit to unlock the keyboard and set its mode int register #1. Control unit 10 then sets aU the registers in the station unit to the desire states according to the operational mode indicated by the station unit. After control uni 10 has set the registers in station unit 18 or 36 to the desired states, control unit 10 wiU only send energy to the station unit when it has something for the station unit to act upon. The idle byte for transmission from control unit 10 is the NO ENERGY signal. Consider now the operation of a station unit 18 or 36. After power-up when the power supply voltage is high enough, the voltage control oscUlator 100 as shown in Figure 2a starts oscUlating and an internal basic clock generator 82 starts. Station unit 18 or 36 uses the 42 kUohertz signal from control unit 10 to phase lock the basic clocks to those within control unit 10. After the power supply voltage is high enough, and a hardware generated time delay period is passed and after the basic clocks are running, a power-on reset (POR) is issued by logic circuit 103 within the chip 60. Power on reset circuit 103 sets the communication logic to a bad frame condition, which means that it is not synchronizing to the frame sync signal, FFF5, transmitted from control unit 10. Power-on reset circuit 103 also initiates a sequence to initialize aU the registers within the register file 58. Power-on reset circuit 103 also loads logic circuitry 59 in chip 60 with the station unit's unique station number and the mode in which it is to operate from extemaUy set switches so the logic knows how to interpret incoming signals and how to configure its inputs and outputs. Power-on reset circuit 103 also starts the timing clocks and sets the transmit data register states to the NO ENERGY level. After operation of power-on reset circuit 103, the communication logic (circuit 59) looks for the frame sync, FFF5, bit pattern in the digital communication channel. If the frame sync is not found within a predetermined time, internal timing is adjusted to differentiate one of seven possible timing combinations. The logic continues to time out every four timing frames and walk to the next timing state until the frame sync bit pattern is recognized. When the frame sync is recognized, communication logic moves to a state caUed not bad frame, NBF. The communication logic continues to look for the frame sync in the time slot where it is expected. If the frame sync is not found in the expected time slot in three tries, the logic goes back to the bad frame condition. If the frame sync is recognized, the logic goes to the good communication condition. The logic assumes the time slot position based on the station number and the station unit is now ready to communicate to the control unit 10. Anytime the station unit does not see the frame sync in the expected time slot, the state of the station unit returns to the not bad frame communication status. Consider now the transmission of data by a station unit. The first thing the station unit sends to control unit 10 is switch inputs B data, then switch inputs A data and then the mode register information from register #0. After that, station unit 18 or 36 wUl send changes to inputs A and inputs B or send the register #0 an idle byte. After station unit 18 or 36 sends a first contents for register #0 , it can receive data from control unit 10. The keyboard of the station unit is inactive until it receives an instmction from control unit 10 to reset the initialization status bit. After the initialization status bit is reset, the keyboard is active and station unit 18 or 36 sends its mode as register #1 for the idle byte. The station unit is now in fuU commumcation with control unit 10 and can receive and send aU types of data. Consider generaUy the reception of data by station unit 18 or 36. When the station unit receives vahd data, that is the bits aU have energy at the plus or minus logic level voltage in its station byte, it wiU react to the data according to that data and past history. For example, if the station unit sent input data at the last opportunity, an echo back flag is set and it compares received data to the sent data. If the data does not compare, the station unit wiU send a NO ENERGY signal to teU control unit 10 that there was an enor and wiU send the conect data again at its first avaUable time slot opportunity. No new data wUl be accepted until conect echo back is received. If the data compares, the echo back flag is reset and the station unit is ready to accept new data from control unit 10 or send additional input data. Turn now to Table 6 which Ulustrates how input data takes precedence over output data in station unit 18 or 36. Table 6 Station Unit Control Unit (ignore output data) < — output data input data > (set echo flag) input data input data < input data (reset echo flag) new input data or idle — - > If any data, use last input data. If new input, echo back at next opportunity. If idle, resend output data at next opportunity Output data could be sent from control unit 10 to station unit 18 or 36. But if the station unit has input data to send to control unit 10, it wiU ignore the received data from control unit 10 unless it is expecting secondary data such as when a secondary register flag is set. If a station unit is clear to receive new data, it tests the most significant bit (MSB) to make sure it is vahd output data and if vahd, it uses the data and echoes the data back to control unit 10 at the next opportunity. If the register field is "8", the station unit recognizes this as the first byte of a 2-byte instmction and sets the secondary register flag. The last four bits of this instmction teUs the station unit where to put the second byte. If the secondary register is reset, and the data is input data (MSB=0) then the data is ignored and a NO ENERGY signal is sent to the control unit at the next opportunity to show an enor as summarized in the depiction of Table 7. Table 7 Station Unit Control Unit idle or echo-back of output data — — > input data < NO ENERGY > NO ENERGY received. If control unit sent input data last time as echo-back, ignore the data. If control unit sent output data last time, resend output data. If control unit sent NO ENERGY last time, count 1 toward off-line If the secondary register flag is set because the station unit is expecting a byte of data, and the received data is vahd, the station unit loads the data into the appropriate register and echoes back the data to control unit 10 at the first opportunity. The secondary register flag is then reset. If the secondary register flag is set and the data is a no energy signal, the station unit resets the secondary register flag and sends a no energy signal to control unit 10 at the first opportunity to show an enor as depicted in Table 8 below. If the echo back flag and secondary register flag are both reset and the station unit receives the NO ENERGY signal, it accepts this as an idle byte and sends the contents of the mode register as an idle byte or sends new input data when it is detected at the next opportunity. Consider now the transmission of data from the station unit according to the priority in Table 5. The highest priority for transmitting NO ENERGY signals enor condition and this can occur when: (1) input data was sent by station unit at the last opportunity (echo back flag set) and new data received does not agree as summarized in the scenario Ulustrated in Table 9; (2) input data (MSB=0) is received as new information from control unit 10 with the echo back flag reset as summarized in Table 7; and (3) secondary data is expected, that is the secondary flag is set, and a NO ENERGY signal is received resulting in the enor condition as summarized in Table 8. Table 8 Station Unit Control Unit < "8 (register number)" "8 (register number)" -> conect echo back Reset secondary flag bit < secondary data regardless of data receivec or NO ENERGY > NO ENERGY or incorrect data <-- NO ENERGY > If idle, resend "8 (register number)". If input data, echo back in next appropriate time slot and resend "8 (register number)" at next opportunity The next highest priority for the station unit is retransmission of data not properly echoed back. This occurs after a NO ENERGY signal has been sent to show an enor so that the control unit 10 knows not to act on the data in enor. The input data wiU continue to be resent until proper echo back is received and the echo back flag is reset. The next priority is the echo back of the secondary register data. This has a high priority because after the transmission of the first byte of a 2-byte instmction from control unit 10, the second byte must be received and echoed back and secondary flag reset to prevent data enors. If the station unit 18 or 36 is expecting the second byte of a 2-byte instmction with the secondary flag set, and NO ENERGY signal is detected, station unit 18 or 36 sends back a NO ENERGY signal and resets the stationary flag as depicted in the scenarios of Tables 8 and 10. Table 9 Station Unit Control Unit input data, (set echo back flag) — > NO ENERGY or inconect data < NO ENERGY > NO ENERGY, ignore last data < resend data > The next priority is the keyboard switch closures. During normal operation, this is the highest priority since switch input data has priority over output data as was depicted in Table 6. This technique is combined with a key stack to prevent loss of data from fast key entries with minimal impact of system response time. The next priority is a switch change data which was discussed above in connection with Table 5. These inputs are for slower response type inputs such as off hook switch, two position switches and the like. The echo back of received output data is the next highest priority. Output data is stored in the memory in control unit 10 and cannot be lost by a delay in being serviced: Therefore, output data is only prioritized over noncritical input data. The commumcation protocol is iUustrated in Table 11 in the case where an enor is echoed back and in Table 11a in the case where output data is enoneously received by the Control Unit. Table 10 Station Unit Control Unit "8 (register number)" inconect data If "8 (register number)" received last time, reset secondary flag < NO ENERGY NO ENERGY > NO ENERGY < — resend output registers* * An enor has been detected by the control unit, aU output registers are restored for the station unit involved. Table 11 Station Unit Control Unit < — output data > NO ENERGY or inconect data NO ENERGY, reset secondary flag < NO ENERGY input data or idle > < — resend output data at first opportunity* * An enor has been detected by the control unit, aU output registers are restored for the station unit involved. The key up command "60" is sent so a tone can be presented to an earpiece as long as a key is pressed. When one key is foUowed closely by another key, switching from one key to the next key tone without intermption is not a problem so that no key up is required. Therefore, if another key is ready to send before the key up is sent, the new key is sent and the key up from the previous key is ignored. FinaUy, the contents of the mode register, register #0 or #1, is sent as an idle byte only if there is nothing else to send from station unit 18 or 36. Station unit 18 or 36 holds aU output data in its registers. These registers are initialized by the power-on reset 103 and are set to other states by control unit 10. If a station unit 18 or 36 goes off-line for any reason, such as being unplugged or plugged back into another location, the output registers are restored by control unit 10. TABLE 11a Station Unit Control Unit < O ENERGY or echo back of input data output data < — resend output data at first opportunity* * An enor has been detected by the control unit, aU output registers are restored for the station unit involved. Voltage Controlled Crystal Oscillator A smaU and economicaUy fabricated CMOS voltage controUed crystal osciUator is provided by coupling three inverter amplifiers in series with a regenerative crystal controUed feedback loop. The first and second CMOS inverters have output nodes whose impedances are modified by a CMOS impedance modulating circuit. Also coupled to each of these two output nodes is a CMOS transistor shunt capacitor. The impedance of the output node is modified according to the magnitude of a voltage control signal appHed to the CMOS modulating circuits. The self-bias of the modulating circuits is maintained substantiaUy constant by adjusting the gate drive in each of the modulating circuits according to gate drives derived from a dummy modulating circuit. The dummy modulating circuit has as one input the voltage control signal and as its compensating gate drive, a control signal derived from a feedback loop from a dummy output node in such a manner that the self-bias of the dummy modulating circuit is maintained substantiaUy constant. The compensating gate drive from the dummy modulating circuit is then coupled to the appropriate compensating gates within the impedance modulating circuits in the voltage controUed oscUlator. The ultimate function of the oscUlator of Figure 7 is to provide a circuit to generate an output voltage with a fundamental frequency which is under the control of quartz crystal 132 and yet can be continuously adjusted above and below the natural resonant frequency of crystal 132 by a slowly adjustable analog voltage, VC. The circuitry of Figure 7 is shown in an implementation which is particularly compatible with CMOS integrated circuit design. The circuit is also of such a nature as to be deposed within a smaU chip area in communication and other types of integrated circuits. The osciUator of Figure 7 forms a phase shifted controUed regenerative crystal oscUlator which osciUates at a fundamental frequency at which the regenerative loop exhibits an overaU phase shift of zero degrees and a loop gain equal to or greater than 1 (or zero decibels). A DC bias is sensed and currents are established in the phase shift transistors in the voltage controUed oscUlator to control phase shift over acceptable limits without saturating the inverter ampHfiers to which they are attached. The components of the circuitry of Figure 7 are fabricated in an inexpensive CMOS integrated circuit and therefore include weU matched components which are inexpensively manufactured. AU the components of the circuit of Figure 7 are fabricated in an integrated circuit form which the exception of the crystal feedback loop generaUy denoted by the elements within dotted outline 180. Figure 7 is a schematic diagram of a voltage control crystal osciUator according to the invention. The osciUator is comprised of three inverting amplifiers 109, 113 and 117, two output impedance modulating stages 121 and 125, two shunt capacitors stages 128a and b and 130a and b, feedback controUer elements 180, an operational amplifier 142, a special inverting control amplifier 150-158, 174, 176 and 178, and an osciUator output buffer amplifier 162. The first amplifier 109 of the three inverting amplifiers is comprised of PFET 108 and NFET 110. The second amplifier 113 of the three inverting amplifiers is comprised of PFET 112 and NFET 114. FinaUy, the third amplifier 117 of the three inverting amplifiers is comprised of PFET 116 and NFET 118. The first of the two output impedance modulating stages 121 is comprised PFET 120 and NFET 122 whUe the second of the output impedance modulating stages 125 is comprised of PFET 124 and NFET 126. The shunt capacitors stages in turn are comprised of PFETs 128a and b and NFETs 130a and b. The feedback control elements 180 comprise crystal 132, resistors 134 and 136 and capacitors 138 and 140. The initial control stage is comprised of operational amplifier 142, output capacitor 144 and summing resistors 146 and 148. The special inverting control gain stage is comprised of operational amplifier 150 in combination with bias tracking stages which in turn are comprised of PFETs 152 and 154 and NFETs 156 and 158 feedback resistor 178 with output capacitor 160. The osciUator output buffer amplifier 162 is coupled to the output of the three inverting amplifiers. The circuit described above in connection with Figure 7 generates an output voltage signal at nodes 164 and 166 with a fundamental frequency which is at or very near the frequency at which feedback crystal 132 is cut or is natural series resonance. The frequency of the output voltage signal at output nodes 164 and 166 is continuously conttoUable within a smaU band of frequencies around the natural series resonance of crystal 132. The fundamental frequency of the output voltage signal of the oscUlator of Figure 7 is continuously conttoUable by slowly varying an analog voltage VC at the input to a summing resistor 146. The first inverting amplifier 109 and second inverting amplifier 113 each exhibit a nominal phase shift of 180 degrees due to their inverting characteristic plus an excess phase lag of approximately minus 45 degrees at the osciUator frequency. The excess phase shift of first inverting amplifier 109 is caused primarily by a lag network comprised of the shunt gate capacitance of FETs 128a and b at node 168 combined with the dynamic output impedance of FETs 108, 110 120, and 122 at node 168. The excess phase shift of the second inverting amplifier 113 is caused primarily by a lag network comprised of the shunt capacitance of FETs 130a and b at node 170 combined with the dynamic output impedance of FETs 112, 114, 124 and 126 which are also coupled to node 170. The excess phase lag of these two inverting amplifiers 109, 113 is therefore approximately minus 90 degrees. An excess phase shift of minus 45 degrees was chosen since this produces the maximum rate of change of phase shift per volt of control, VC. The exact amount of excess phase lag can be adjusted by analog control voltages appHed to the gates of FETs 120 and 122, and 124 and 126 which modulate the net dynamic output impedance at nodes 168 and 170 respectively. The particular excess phase shift of each of these two inverting amplifiers is adjusted so that the overaU phase shift around regenerative loop of the osciUator in Figure 7 is at the desired frequency of osciUation, but is sufficiently close to the natural series crystal frequency to aUow the crystal to maintain control. Control with the use of two stages was selected to yield a larger phase shift in degrees per volt thereby extending the useful control range of the design. At DC and low frequencies, the feedback from output node 164 to input node 172 is degenerative and therefore the osciUator of Figure 7 is self biased to a Class A condition, thereby ensuring that osciUation wUl always buUd up at any higher frequency in which the overaU phase shift is zero degrees and the voltage gain is equal to or greater than zero decibels. When the control voltage, appHed to the gates of FETs 120 and 124 goes more negative, a control voltage appHed to the gates of FETs 122 and 126 must go more positive by just the right amoimt so that the drain cunent of FET 120 is equal to the drain cunent of FET 122 and simUarly the drain currents of FETs 124 and 126 are equal to produce essentiaUy no displacement current at nodes 168 and 170 respectively. Maintaining essentiaUy zero displacement current at nodes 168 and 170 is necessary to sustain self-bias of the overaU oscUlator loop independent of the analog conttol voltage, VC, which is appHed to summing resistor 146. The balance of these currents is realized using an auxUiary pair of FETs 174 and 176 which have operational characteristics simUar to control FETs 120, 122, 124 and 126. The common drain voltage of auxUiary control pair 174 and 176 is maintained at a centering voltage of approximately zero volts by developing an unique voltage at the gate of FET 176 to just balance the drain currents of the two FETs 174 and 176. The conect balancing voltage is provided by operational amplifier 150 in combination with feedback resistor 178. The input nodes of operational amplifier 150 are biased to the same self-bias as realized by the overall osciUator loop using FETs 152, 156, 154, and 158 which in turn have operational characteristics similar to FETs 108, 110, 112 and 114. The use of MOS FET transistors 128a and b and 130a and b as the phase shift capacitors within the circuit aUows for better access phase shift control because the gate capacitance of MOS FET transistors 128a and b and 130a and b can be manufactured to match the gate capacitance of the input FETs 108 and 110 respectively. The use of shunt control elements 120, 122, 124 and 126 to modulate the dynamic output impedance of the amplifier also aUows for a good phase shift control over a large signal swing. The combination of a simple amplifier as typified by amplifier 109 with modulation of its output impedance by FETs 120 and 122 and with fixed capacitors 128a and b as loading elements to form a control phase shift amplifier is particularly weU implemented in CMOS integrated circuit design. The osciUator of Figure 7 is also particularly characterized by the use of two voltage control stages to extend the useful control range of the osciUator. The network in Figure 7A can be substituted for resistor 134 and capacitor 138 which improves the performance for some characteristics of ampHtude and phase shift of inverting amplifiers 109, 113, and 117 and associated circuitry 121, 124, 128a and b, and 130 a and b. Light Emitting Diode Driver Circuit A low power, low noise driving circuit for a bank of LEDs utilized in the station units of the MAN system is provided by coupling each bank of LEDs in a series circuit between the voltage supply and a constant current source. Each LED has a controllable logic switch in paraUel across it and the switches are further in series circuit with each other to form a ladder network. Any selected LED may be turned off by closing its conesponding logic switch. The cunent continues to flow then through the shunting switch into the remaining LEDs in the series circuit that are on. A plurahty of such ladder networks may be coupled in paraUel with each other and each ladder network controUed by a switching gate which selectively couples it to the constant current source so that the LED ladder networks are operated at a predetermined duty cycle. Current spikes are avoided across the voltage supply by driving the connecting control gates in an overlapping relationship so that the constant cunent source is never disconnected from the voltage supply. Turn to Figure 8 which is a simplified schematic of a first embodiment of the invention. A supply voltage V is coupled to a serial bank of LEDs 180, 182 and 184. Coupled in paraUel across each LED 180-184 is a logic switch 186, 188 and 190 respectively. The bank of LEDs 180-184 is coupled to the opposing raU of the voltage supply through a constant current source 192. When switch 186 is closed, current flows through switch 186 and LED 180 is turned off since the voltage across LED 180 is close to zero volts and so below its threshold voltage. When logic switch 186 is opened, aU the cunent flows through LED 180 and it emits light. When switches 186-190 are aU open, the same current flows through each of the LEDs 180-184 and they aU emit Ught. For most cunent sources, the voltage at node 194 between the bottom LED 184 and cunent source 192 must be some minimum voltage above the negative voltage of the voltage supply in order for the cunent to remain relatively constant through current source 192. The maximum number of LEDs that can be placed in series is then determined by the coUective maximum voltage drop of the LEDs and the difference between the voltage supply V and the minimum voltage VI at node 194. If some LEDs cannot logicaUy be on at the same time, then more LEDs can be added to the number of LEDs in series, but the resistance of the switches 186-190 must be considered in the design. If the switch impedances do not add to the total voltage drop as would be the case in a circuit as realized and shown in Figure 8 (i.e. the worst voltage drop being with aU LEDs on and the "not aU on at once" option above is not used to increase the number of serviced diodes), then switches 186-190 need only to keep the voltage across each conesponding LED to a level that does not emit visible Ught and can therefore be a relatively high impedance switch. Figure 9 is a schematic diagram of a MOS FET embodiment for implementation in an integrated circuit conesponding to the schematic diagram of Figure 8. The analogous circuit portions bear the same reference numerals. Constant cunent source 192 is comprised of a resistor 196 and two NFETs 198 and 200. The specific design of the constant cunent source circuit 192 is not material to the invention and therefore wiU be not further described. Each switch 186, 188 and 190 is comprised in turn of MOS FET bUateral transistors 202 and 204 with common connected sources and drains across the gates of which is coupled an inverter 206. Again, the specifics of the switching element as shown in Figure 9 are not material to the invention and therefore are shown in Figure 9 only Ulustratively. The circuitry in Figure 9 for example is a CMOS circuit but the circuit could be easUy adapted to be implemented either as an NMOS or PMOS design if desired without departing from the scope of the invention. Figure 10 is a schematic of a circuit simUar to that shown in Figure 9 with the exception that the circuitry of Figure 10 is adapted to operated at a 50 percent duty cycle whUe the circuitry of Figures 9 and 8 is designed to operate at a 100 percent duty cycle. In other words, added to two identical switch and LED banks are control gates 208 and 210 which are driven by complementary square wave drive signals such that both banks of LEDs are gated on only for a short overlapping time at the beginning and end of each clock cycle. At aU other times the LED banks are alternately on or off. The resistance of gates 208 and 210 is important in determining the number of LEDs that can be coupled in series and should be relatively low. The time overlap between the control signals, Tl as shown in diagrammaticaUy in the timing diagram of Figure 11, is important because if gates 208 and 210 were both simultaneously off, a cunent spike would occur at the minus or plus voltage supply nodes. A circuit simUar to that shown in Figure 10 can be constructed and operated in an analogous manner with other conesponding duty cycles simply by adding additional banks of LEDs and switches and conesponding gate controls in the same manner as Figure 10 extends the circuitry of Figure 9. In other words, to accommodate three LED and switch banks a 33-1/3 percent duty cycle can be needed, the addition of four LED and switch banks wUl require a 25 percent duty cycle and so forth. Driving a series bank of LEDs using a constant cunent source has the advantage of providing consistent Hght emission from the LEDs with lower maximum cunent used in the circuit. Switching the LEDs in series using paraUel logic switches permits complete display flexibUity without changing the current demands or creating current spikes in the power supply lines. The circuitry shown in concept in Figure 8 can be simply implemented in integrated MOS FET form as depicted by the examples in Figures 9 and 10. The methodology can be easUy adapted to a wide variety of duty cycles and series diodes banks, some of which are shown in the examples of Figures 9 and 10. The variable duty cycles can be implemented without causing cunent spikes in the power supply lines by utilizing gating clocks which include an overlap in the control signals. If because of the particular appHcation the possibiHty arises that any one of the LEDs would logicaUy never be on at the same time as selected other ones, the number of LEDs in series circuit can be increased even beyond what which might otherwise be tolerated by constant cunent source 192, however switch resistance voltage drops need to be considered in such cases. It should be evident that the circuit design of Figures 8- 11 is also readUy adaptable to bipolar transistor design. Automatic Timing Compensation for a Communication Line In the iUustrated embodiment, the system is shown in Figure 1 is comprised of control unit 10 coupled to a twisted wire pair 20 which in turn is coupled to a plurahty of remote station units 18 or adapter boxes 36. Control unit 10 contains all the logic for controlling the communication on line 20. Adapter boxes 36 can be treated as the same as station units 18 when considering and discussing automatic timing compensation. Control unit 10 and station units 18 and 36 are synchronized to a common reference signal which sets their receive timing. The transmit timing is then adjusted so that regardless of the amount of line connected within the aUowed limits of Hne length, the signals wiU be received by control unit 10 and each of station units 18 and 36 at approximately the same time relative to the receive timing regardless of the point from which they were transmitted. The main component or cause of delay time is the line capacitance of line 20. When the system of Figure 1 is initialized, a transmit advance signal or time is preset according to the invention to accommodate a line that is near the middle of the permitted Hne length and of average type. The effective capacitance is then measured by circuitry within control unit 10. The adjusted advance time is then set in the control unit 10 and communicated to each of the station units 18 and adapter box 36. Once the transmitted advance time of station units 18 and adapter boxes 36 are adjusted, the system of Figure 1 is optimized for delay characteristics of the line. line 20 is then tested and readjusted periodicaUy to compensate for any changes which may have occuned to the capacitance on line 20 such as the addition or removal of stations, wire, or adapter boxes from it. The solution of the invention provides an improved performance over a fixed advance time as is common in the prior art and can be easUy integrated with other circuitry to provide a low cost solution to the problem. In addition, an advance timing misadjustment can be detected at control unit 10 and can be estabUshed for each separate station unit 18 and 36 and the transmit advance time for each station unit 18 or 36 can be finely tuned either using a static adjustment or a dynamic setting dependent upon the communication task at hand for each station unit 18 and 36. The implementation of the invention is iUustrated in the schematic diagram of Figures 12 and 13. In Figure 12, the transmitted advance time or data appears on data bus 42 after being communicated from control unit 10 as described in connection with the block diagram of Figure 1. In the iUustrated embodiment of Figure 12, register C is used for storage of the transmit advance time Register C is one of many contained in the MAN chip of Figure 2 in its register file 58. In particular, bits 4-7 are used to carry the transmitted advance time. The transmitted advance time is then written into register bits 214-220 included within register file 58 of Figure 12 as controUed by decoder 212 and by the register write command, REGCWRT. The contents of registers 214-220 are then avaUable to the internal logic within the circuitty of Figure 2 on discrete control and logic lines 222. Turning to Figure 13, these control signals are now avaUable in logic control circuitry included within control circuit 59 of Figure 2a to be used in conjunction with a plurahty of timing signals to adjust a timing control signal, TRES at output node 224 which is used in the internal control logic of control unit 10 of Figure 1 to determine the difference between the transmit time of data from the station unit to the receive time. Inasmuch as the circuitry of Figure 2a is shared in common between control unit 10, station units 18 and adapter box 36, the control and timing circuitry described in connection with Figures 12 and 13 is also equaUy appUcable and avaUable for use in connection with station-to-station or station-to-adapter-box communication. The detaUs of the circuit diagram of Figure 13 have not been described since they are largely immaterial to the concept and understanding of the invention, although they are iUustrated in the Figures in a specific embodiment. A Low Cost Adaptive Echo Balance Methodology The invention is an interface to a communication Hne which compensates for various line lengths, the type of line and other devices coupled to the line. The circuit operates to attenuate echo caused by these and other line variations. The technique described is adaptive and can be updated periodicaUy to adjust for a device being added or deleted from the line and other line lead changes. Turn to Figure 14 wherein a block diagram of the iUustrated implementation of the invention is iUustrated. A control unit 10a is provided such as a control unit of a key telephone system which controls all the signaling and interfaces to central telephone office lines 12 in Figure 1. Control unit 10a is coupled as wiU be described below to station unit 18a which for example may be a key telephone handset. Control unit 10a and station unit 18a play much the same role as control unit 10 and station unit 18 or 36 in Figure 1, except that the additional circuitry of Figure 14 is contemplated as being included within control unit 10 and station unit 18 or 36 in Figure 1. A test tone generated at 270 on conttol unit input 274 is introduced on commumcation line 20 from conttol unit 10a with aU stations 18a connected to Hne 20 but off-line. An RC network is adjusted until best balance is achieved or in other words the least echo received back. Only one station unit 18a has been depicted in Figure 14 for the sake of simpHcity but it is expressly understood that multiple number of station units or adapter boxes -36 as described in connection with Figure 1 may also be coupled to line 20. As described in greater detaU below, the receive signal is then conditioned through a peak voltage detector 228 and into an analog-to-digital converter 230. An analog-to-pulse width converter may be substituted for the analog to digital converter for lower cost. A processor 232 or other custom logic circuit is used to change the resistive and capacitive parameters coupled through balance network 226. The echo back from the tone is then once again received and tested to determine if an improvement has been realized. After a best value for the signal or least echo has been established, each station 18a is signaled to adjust balance network 226 located in the station unit to be set at the same or simUar values. In the iUustrated embodiment as schematicaUy depicted in Figure 15, balance network 226 is comprised of a pluraUty of capacitors 234 of different values, shown in the block diagram of Figure 14 as being C, 2C, 4C and 8C, and a plurahty of switched capacitor resistive equivalent elements 236, again shown as R and 2R in Figure 14. Each of the elements 234 and 236 are switched through conesponding switches S1-S6 coUectively denoted by reference numeral 238 to an output node 240. Switches S1-S6 are implemented as conttoUable semiconductor switches, but are schematicaUy shown in Figure 14. In the iUustrated embodiment, capacitors 234 are tested in various arrangements and set to an optimum combination to reduce the echo. Thereafter, resistors 236 are likewise tested and combined to further minimize the echo. The values of capacitor and resistors are weighted so that sixteen values of capacitors are avaUable in equal steps and four values of resistors are avaUable in equal steps. A specific implementation of balance network 226 is iUustrated in CMOS circuitty in the schematic of Figure 15. Figure 15 shows switches 238 implemented with CMOS switches using an inverter 242 in combination with a pair of CMOS transistors 244. Resistors 236 are implemented using switched capacitor equivalent. An echo balancing circuit 246 is utilized in Figure 14 which is coupled between line interface unit 248 and control unit 10a on one hand and between line interface unit 248 on the opposing end of communication line 20 and station unit 18a on the other hand. The detaUs of echo balancing circuit 246 are better depicted in the schematic of Figure 16. Nodes 250 and 252 can be considered as inputs to a summing amplifier 264. Node 254 is a virtual ground so that when cunent in resistor 256 from node 250 to the virtual ground 254 equals the complement of the current in resistor 258 between node 252 and virtual ground 254 for the echo signal, echo canceUation is achieved. The combination of resistor 260 and the balance network are used to attempt to match the delay in ampHtude of the echo signal on node 252 which is coupled to the incoming received signal through line interface 248. Resistor 260 is coupled across the output of the transmitted signal from control unit 10a to line interface 248 and to node 250 which is also coupled to balance network 226. Ideally there should be 180 degree phase shi between the transmit signal received on node 262 and the echo signal received on nod 252. When the capacitors of balance network 226 are adjusted, the phase delay at nod 250 is directly effected. When resistors 236 of balance network 226 are adjusted, th ampHtude at node 250 is adjusted. The values thus chosen for the elements withi balance network 226 must be large enough to adjust the worst variations expected i communication line 20. The accuracy of canceUation that is achievable wiU based upo the minimum value for the elements within balance network 226. Any imbalance of signal at node 254 is amplified by operation amplifier 264 an coupled into the receive terminal, RXI, of conttol unit 10a. This value is coupled the to peak detector 228 and a peak voltage converted in analog-to-digital converter 230 t an input port or latch 266. Adjustment is made by processor 232 to minimize the echo i the adjusting signal output on data bus 268 to control unit 10a and thence to switche 238 within balance network 226. Circuitry within the control unit 10a conditions the analog signal received fro amplifier 246 and put it out on terminal 272. If Hkewise conditions the analog test signa from osciUator 270 which is transmitted onto communication line 20 for purposes o echo canceUation. As control unit 10a is part of a key telephone system, terminals 27 and 274 of control unit 10a is also coupled to a line interface unit connected to th central office lines (not shown). For the purpose of Ulustration, osciUator 270 is show as coupled to input 274 directly, but in a practical embodiment, the test tone is switche into terminal 274 only during the adjustment period. Control unit 10a also is used to control the interface signaling informatio between processor 232 and station units 18a. The signaHng information can either b done on separate lines or as in the iUustrated embodiment, multiplexed with the analo data over common communication line 20. In the diagrammatic depiction of Figure 14 station unit 18a includes analog circuitry to condition the analog signal coming in on th receive node, RXI, and outputting it on the audio output node, RA Station unit 18 also receives an incoming audio signal on input node TA and outputs it to node TXO In a key telephone system, the TA and RA nodes would be coupled to a handset. I addition, station unit 18a wiU contain circuitry to control the interface signalin information between control unit 10a and station unit 18a and to use the signalin information furnished by control unit 18a to set its own conesponding balance networ 226. line interface units 248 provide the interface between a two-wire communicatio line having data flowing in both directions and separate transmit and receive signal wire with a common reference ground wire. A simple representation of line interface uni 248 is shown in the block diagram of Figure 17. As depicted, interface unit 248 include two buffers or amplifiers for transmitting information in opposite directions, from communication line 20 to transmit and receive lines TXO and TXL This line interface unit is a typical two-to-four wire conversion circuit and is used at each termination of a telephone line. Peak detector 228 first determines the peak voltage on the signal input to it and generates a steady analog output voltage in response as a measure of the peak input voltage. Digital to analog converter 230 is a conventional 8 bit analog to digital converter. Processor 232 performs the functions of outputting test values to balance network 226, reading the value of the output of A-to-D converter 230 through an input port 266 and thereby determining and setting the optimum value in balance network 226 before the ending of the echo balance routine. The echo balance routine is performed at startup and occasionaUy updated, typicaUy at off peak periods of processor and system activity. A Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled, Communication Line Interface An improved interfacing system is provided for coupling a plurahty of sources to a two-wire communication pair. The interfacing system is comprised of a plurahty of differential voltage-to-cunent amplifiers which are capacitively coupled to the Hne pair. Each differential voltage-to-cunent amplifier includes two complementary cunent sources, one for driving cunent into the defined balanced DC ground of the communication line and the other for drawing cunent from the ground of the DC balanced communication line. Both current sources are voltage controUed from the communication source. A differential receiving amplifier is RC coupled to the communication line and is used as a common mode rejecting input amplifier. An echo balancing network is coupled to the output of the differential receiving amplifier and the echo canceUation signal is provided from the source being coupled to the communication line. Each source is provided with its own conesponding differential voltage-to-cunent amplifier and echo balancing network whUe the differential receiving amplifier used to receive signals form the line is coupled in paraUel and shared among aU the source and receiving entities coupled to the communication line at that particular interface. Turn to Figure 18 which shows an interface from a single sourcing and receiving entity to an internal communication line 20. The interface, generaUy denoted by reference numeral 666, is comprised of among other things a differential voltage-current amplifier 668 which is a capacitor coupled differential driver amplifier which converts a single ended input voltage to a differential pair of complementary cunent source outputs. The voltage-to-cunent conversion for one phase is provided by differential amplifier 670 and its conesponding resistors 672-680 and capacitor 682. The voltage-t current conversion for the opposing phase is similarly provided by differential amplifie 284 with its conesponding resistors 286-294 and capacitor 296. A capacitor coupled differential receiver amplifier, generaUy denoted b reference numeral 298, converts the differential line input voltages on lines 300 and 30 to a single ended output voltage. Receiver amplifier 298 is comprised of operatio ampHfier 304 and its conesponding resistors 306-312. An echo canceUation stage 314 is provided at the output of capacitor couple differential receiver amplifier 298. The components within echo balancing circuit 31 are identical to and have been described above in connection with the circuitry Figures 14-16 and include circuitry equivalent to balance network 226 and summin amplifier 246. The output of echo balance circuit 314 is provided as an input signal t the circuitry for which circuit 266 serves as an interface. Capacitor 316 on line 300 and capacitor 318 on Hne 302 are isolating capacitor to provide DC isolation from communication line 20 as weU as the AC signalin connection to the same line. Resistors 320 and 322, which are equal in resistance valu establish a ground reference for the circuit 666 and other local circuitry. Resistors 32 and 322 are kept at as high an impedance as is practical. The cunent drawn by the interface circuitry and any other circuitty drawin cunent from the line must be balanced. In a telephone system this interface should b used for aU system telephones and all adapter boxes that interface between intern communication line 20 and commercial telephone equipment or any other remot station type instrument. Consider now the operation interface 666. Differential voltage-to-cune amplifier 670 operates with more cunent flowing to signal ground with a positive chang in its input voltage, whUe voltage-to-cunent amplifier 284 operates to flow more cune from signal ground 324 with a positive change in its input voltage. It can be show through analysis of the circuit in Figure 18 that the current output by voltage-to-cune ampHfier 668 is proportional to the input voltage as determined by a proportionalit constant only dependent upon values of the internal resistors. Since the input voltage are appHed antisymmetricaUy with respect to differential ampHfiers 670 and 284, th phase and cunent supphed by each subcircuit conesponding to amplifiers 670 and 28 to the line load relative to ground node 324 is equal and opposite. This, therefore, provides a voltage driven cunent source as the interface driver t internal communication line 20. Differential receiver amplifier 298 and echo balanc circuit 314 are used in combination with the above circuits in interface 666 to efficientl cancel the echo back signal from the driver output on line 20 and provide signal bac form the line to the circuitry being interfaced . The schematic of Figure 19 Ulustrates a circuit wherein a plurality of interfaces are coupled to communication line 20. A plurahty of voltage-to-cunent converters 668 are coupled in paraUel to lines 300 and 302, each provided with its own conesponding echo balance circuit 314. A single differential receiver amplifier 298 is coupled across lines 300 and 302 for each of the echo balance circuits. In this way the receive circuitry, isolation capacitors and ground reference resistors are shared in a multiple bank interface. In the iUustrated embodiment the interface of Figure 19 is used in a control unit or master telephone where outside lines are interfaced to internal communication Hne 20. As iUustrated in Figure 20 at some point within internal communication line 20 a low AC impedance circuit, generaUy denoted by reference numeral 326, is provided across communication lines 20. Impedance circuit 326 is comprised of capacitors 328 coupling resistors 330 to a signal ground 332. Circuit 326 is used to provide a balance load to a sohd ground and to minimize the effects of capacitance and resistance variations on communication line 20 In addition, circuit 326 estabUshes a return to ground for Hghtning protection. Circuit 326 may, for example, be instaUed across lines 300 and 302 between capacitors 316 and 318 at the outputs of the differential voltage- current amplifiers 668 in the system as depicted in Figure 19 when the circuitry in Figure 19 is located in the same location where the hard ground is estabhshed. This would aUow the stored use of capacitors 316 and 318 in place of capacitors 328 as weU as eliminating the need of the high impedance resistors 320 and 322 in favor of the low impedance resistors 330 in that case. In addition as shown in Figure 20, somewhere on communication line 20, a DC voltage can be estabhshed through inductors 334 and 336 to provide power to the line. Inductors 334 and 336 are used to minimize the power supply loading effects on the signals. The power supply reference is estabhshed relative to ground 332 connected to resistors 330. There are system advantages to combine the power suppHes, low resistance resistors 330 and the power consuming digital control circuitry within a single interface although this is not required. The DC power suppHes can then be used to power digital circuitry, estabhsh a reference for interfaces, that include a line power supply and are used to supply aU the power for the remote line powered interfaces. The circuit described has many advantages, the chief of which are described as foUows. The circuitry of Figures 18-20 aUow a multipHcity of sources to be coupled to internal transmission line 20 at the same time without the bulk size, cost, and poor characteristics of a transformer interface. The grounds of the various sources are isolated by the relatively high impedance provided by resistors 320 and 322 for each source. The system can operate with a relatively large common mode voltage. Differential DC voltage can be placed across internal communication line 20 for the distribution of DC power. The use of a balanced AC differential cunent output a described in Figure 18 provides a high DC impedance that aUows many sources to b coupled to line 20 simultaneously without serious degradation of the communicatio signals. The use of a relative low impedance balanced load circuit 326 as shown i Figure 20 which is located only at one position within the system in combination with a interface circuit having relatively high impedance aUows for the possibihty of man interfaces to be coupled to line 20 with large variations in line length without sacrificin the signal integrity. The use of circuitry that has a balanced DC load and a DC powe source provided through inductors 334 and 336 aUows Hne powering ove communication line 20 without lowering the AC impedance of each interface o disturbing signal cunent information. The use of a pair resistors in circuit 326 alon with a balance load aUows for the setting of a good ground reference while maintainin high source impedance which in turn aUows many interfaces to be coupled to line 2 without significantly lowering the impedance of the line. The use of an adaptive ech balance circuit 314 as part of interface 666 aUows large variations in bot communication line length and the number of interfaces coupled to communication lin 20 without causing large echo signals which in turn generate communication enors. Th sharing of differential circuit 668 for multiple sources as shown in Figure 19 minimize the amount of hardware, power and line loading that occurs. Further, the use of differential ampHfier 304 to provide the input signals to the sources provides substantia common mode rejection. Telephone Line Interface An improved telephone line interface unit for coupling through a transforme into telephone office transmission lines and equipment is provided by first amplifyin the audio signal from its source and then inverting the amplified audio signal befor coupling it through a load resistor in series with the secondary of the transformer that i coupled to the telephone lines. The amplified signal from the first amplifier is the coupled through a balance network to the input of the input amplifier also coupled t the secondary of the coupling transformer. The balance network simulates th impedance of the central office telephone equipment, central office transmission Hn the source resistance of the central office, the magnetizing inductance of the couplin transformer, the load resistance, and the effects on low frequency gain of the inpu amplifier by the winding resistance of the transformer and the low frequency cut o characteristics of the transformer. The inverted audio signal coupled to the load resisto is then summed with the feedback signal coupled to the balance network to provid substantial echo reduction through the input amplifier also coupled to the transformer. The telephone line interface unit of the invention is schematicaUy depicted in Figure 22. As before, telephone lines 12 are coupled through transformer 338 and load resistor 352 to provide a 600 ohm termination as seen from the telephone central office line. The audio output signal, RA, coupled to node 354 from the internal circuitry of the equipment being interfaced is coupled to an output summing amplifier stage comprised of operational ampHfier 356 and its associated summing resistors 358, and 3 kHz upper baseband roU off capacitor 360 and the 300 Hz lower baseband roU off capacitor 362. The output signal from operational amplifier 356 is further amplified and delivered to telephone line 12 through load resistor 352 and transformer 338 by means of an inverting amplifier stage comprised of operational ampHfier 364 and summing resistors 366. The input audio is received by a summing amplifier 368 and its conesponding summing resistors 370 with a 3 kHz upper baseband roU off capacitor 372 and a 300 Hz lower baseband roU-off capacitor 374. Summing amplifier 368 also receives the audio output signal from the internal circuitry through a summing resistor 376 as altered by balance network 378 which is depicted and described in greater detaU in connection with Figure 23. The audio signal input to balance network 378 and summing resistor 376 is out of phase or inverted with respect to the audio output signal coupled to the input transformer 338 via load resistor 352. In this manner echo canceUation or echo balance of signals is provided so that the input wiU not receive its own output signal. Turn now to Figure 23 which is a schematic diagram of balance network 378. It is the purpose of balance network 378 to form an audio output signal whose ampHtude and phase wiU cancel the audio output signal formed at output node 380 of transformer 338. An assumption is made that the normaUy expected type of telephone central office line and telephone central office equipment is connected to the input terminals of transformer 338. Balance network 378 in Figure 23 comprises an equivalent circuit to the telephone central office source resistance 382 and four lumped equivalences of a typical transmission telephone line comprised of resistor and capacitor pairs 384-390. In the depiction of Figure 23 the resistance of pair 390 also includes the equivalent of the DC winding resistance of transformer 338. A low frequency cut off capacitor 392 is then coupled in series with the equivalent impedances. Low frequency cut off capacitor 392 compensates for the effect of the magnetizing inductance of transformer 338. Series coupled resistor 394 is provided to compensate for the source resistance of load resistor 352. The ratio of resistor 396 and 376 compensate for the low frequency effect on the gain of operational ampHfier 368 caused by the transformer winding resistance. The low frequency cut off associated with the magnetizing inductance of transformer 338 is compensated by capacitors 398. The overaU effect of the circuitry Figure 22 with the balancing network of Figur 23 is to provide a means whereby a suitable interface is obtained between telephon central office line 12 and equipment coupled to the line with good echo canceUatio The balance network 378 simulates a typical central office line and the couplin transformer characteristics in order to achieve good echo canceUation at low cos Operational ampHfier stages 356 and 364 are utilized to drive load resistance 352 and t invert the output signal to thereby aUow summing into a simple feedback network to b an effective form echo canceUation. A Piezo Driver Using Voltage Doubling and CMOS Techniques An improved driver for a piezo element is provided by driving two CMOS driv circuits out of phase with respect to each other by the signal used to control the piez element. The output ampHtude of each CMOS driver is equal to the difference betwee the voltage supply across the CMOS driver. The out-of-phase outputs from the tw CMOS drivers are then coupled across the piezo element to provide twice the voltag ampHtude change to drive the piezo element with the effect that increased audio volum is generated without the necessity of providing a higher voltage supply terminal to th circuit. Turn to the schematic of Figure 24 wherein a piezo driver of the invention i depicted. When node 400 is high, nodes 402, 404 and 406 wiU be low while nodes 40 and 410 high. The signal at node 400 is coupled through an inverter 412 to the gates PFET 414 and NFET 416. The output of FETs 414 and 416 is common node 408 whic in turn is coupled to the gates of PFET 418 and NFET 420. The output of FETs 41 and 420 is node 404 which is coupled to one terminal of piezo element 422. SimUarly, node 400 is coupled to the gates of PFET 424 and NFET 426. Th output of FETs 424 and 426, node 406, in turn is coupled to the gates of PFET 428 an NFET 430. The output of FETs 428 and 430 in turn are coupled to node 410 which i coupled to the opposing terminal of piezo element 422. When node 400 is high, the voltage across piezo element 422, namely the voltag across nodes 404 and 410, is equal to the negative of the difference between the positiv and negative supply voltages Vp and Vn. When node 400 is low, the voltage acro nodes 404 and 410 is equal to the positive of the difference between the positive an negative supply voltages Vp and Vn. Therefore, the change across piezo element 42 for each transition in the voltage at node 400 is 2(Vp-Vn). Piezo element 422 wi therefore have an audible volume that is the same as that which would be generated b turning on and off a voltage with the magnitude of 2(Vp-Vn). No high voltage power supply is required in order to achieve this voltag doubling. Therefore, the circuit of Figure 24 provides an increase in voltage chang across piezo element 422 to improve the volume of its sound output and it is done with circuitry which is CMOS. Therefore, a low cost and effective piezo element driver is provided by using two CMOS drivers operating in opposite phases to drive piezo element 422. Voltage Limiter Improved linearity of an ampHfier stage in its useful dynamic voltage range is achieved whUe providing voltage limitation of the amplifier output by utilizing two series circuits in paraUel across the input and output of the amplifier. Each series circuit is comprised of a series of bipolar ttansistors, one series being arranged and configured with respect to the output of said amplifier to limit the negative voltage excursion, whUe the other series circuit is anange and configured with respect to the output of said amplifier to Hmit the positive voltage excursion. The number of bipolar transistors coupled in series within each leg is used to determine through the coUective series base- to-emitter threshold voltages the output voltage limitation range for the amplifier. Figure 25 is the schematic of a circuit for providing extended linear dynamic range for voltage limitation on an amplifier. An operational amplifier 432 is provided with an input 434 into which an input signal Vi is appHed through an input load resistor 436. A feedback resistor 438 is coupled in paraUel between input node 434 and output node 440. Resistors 436 and 438 together with operational ampHfier 432 comprise a conventional inverting ampHfier stage. Without limiting operation, input node 434 is a high impedance virtual ground and the cunent through the feedback resistor 438 approximately equals the cunent through input resistor 436 with the voltage gain of amplifier stage 432, 436 and 438 equal to the negative of the ratio of the resistances of resistor 438 to resistor 436. Also coupled in paraUel across input nodes 434 and output node 440 is a transistor 442 in series with transistor 444 in one leg and transistor 446 in series with transistor 448 in a second leg. When the output voltage Vo, at node 440 becomes more positive than the sum of the base to emitter voltages through transistors 446 and 448, transistors 446 and 448 wUl turn on and shunt the cunent around feedback resistor 438, thereby limiting the output voltage. The base and coUector of transistor 448 are coupled in common and the emitter of transistor 448 is coupled in common to the base and coUector of transistor 446. The emitter of transistor 446 is coupled to input node 434. Similarly, the base and coUector of transistor 442 are coupled in common to input node 434 and its emitter coupled in common to the base and collector of transistor 444. The emitter of transistor 444 is coupled to output node 440. After transistors 446 and 448 start to turn on, the excess current is controUed by the threshold parameters of these transistors and gives a sharp knee 450 to the voltage gain of the amplifier of Figure 25 as shown in the voltage gain graph of Figure 2 Similarly, when output voltage, Vo, becomes more negative than the base to emitte voltage of transistors 442 and 444, the negative voltage of the amplifier of Figure 25 i similarly limited. Figure 26 also Ulustrates the difference between the transistor voltage limitatio of Figure 25 as compared to a prior art conventional diode limitation as depicted i dotted line 452. It can readUy be seen that for a given voltage limit, the transisto limiting scheme of the invention gives a much more linear dynamic range than could b achieved by prior art diode Hmiting. In the present embodiment, two transistors 446 and 448 or 442 and 440 are show in series to limit the output voltage to swings of about 1.0 volts. The output voltag swing can be limited to a higher voltage or lower voltage by placing more or fewe transistors in series than those depicted in Figure 25. Therefore, the invention must b understood to include any number of limiting transistors as may be desired. Further, although the iUustrated embodiment shows NPN transistors being used it is also expressly contemplated that PNP bipolar transistors may be used as weU. B the use of bipolar transistors to limit the output voltage swing on an amplifier, voltag limitation can be obtained with good linear dynamic performance over the usefu voltage range. StUl further, it is expressly contemplated within the invention that the feedbac path utilized in the amplifier need not be the simple resistive feedback provided b resistor 438 but any more complex feedback scheme can be substituted withou materiaUy altering the scope of the invention. Line Powering for Two-Wire Twisted Pair, Multiple Signal, Capacitor Coupled Communication Line Interface A supply having multiple supply voltages is provided to supply power on a two wire communication line without the injection of interfering noise onto the line b providing line power interface circuits in both powered line interfaces and nonpowere Hne interfaces. In the powered line interface, a constant cunent source and adjustabl current source are combined to provide a floating ground in the power supply which i centered between the voltages on the two-wire communication line. The adjustabl current supply is adjusted according to the off-center drift sensed though an operationa amplifier so that its current output matches the current of the constant current source t thereby maintain the ground reference signal in a centered position. Security o connection of the line interface units is provided by coupling the interface units to th two-wire line through a diode bridge to ensure that a predetermined polarity is alway provided to the line interface. In the nonpowered interface, a fixed constant cunen source is provided to maintain the diode bridge in a forward biased linear region of operation and to appropriately load a resistive voltage divider to provide and center the floating ground and to provide a proper voltage bias across electrolytic capacitors to which the two-wire communication line is coupled. A supply having multiple supply voltages is provided to supply power on a two- wire communication line without the injection of interfering noise onto the line by providing line power interface circuits in both station unit line interfaces and adapter box Hne interfaces. In the station unit line interface, a constant cunent source and adjustable current source are combined to provide a floating ground in the power supply which is centered between the voltages on the two-wire communication line. The adjustable cunent supply is adjusted according to the off-center drift sensed though an operational amplifier so that its cunent output matches the cunent of the constant cunent source to thereby maintain the ground reference signal in a centered position. Conect connection of the line interface units is provided by coupling the interface units to the two-wire line through a diode bridge to ensure that a predetermined polarity is always provided to the line interface. In an adapter box, a fixed constant cunent source is provided to maintain the diode bridge in a forward biased linear region of operation and to appropriately load a resistive voltage divider to provide and center the floating ground and to provide a proper voltage bias across electrolytic capacitors to which the two-wire communication line is coupled. The line powering signal interface has been described in detaU above in connection with Figure 20. To reiterate, the line power source is comprised of two large coUs 334 and 336 connecting DC power sources + V and -V to a positive line 500 and a negative line 502 which are also named Hne 20 for signal purposes. CoUs 334 and 336 are of like value, manufacture and construction and supply voltages which are set to the same but opposite polarities. While matching values for coU 334 and 336 are not necessary, matching is desirable to maintain symmetry and balance of lines 500 and 502. In the iUustrated embodiment, coU 334 and 336 have an inductance of 330 millihenries and are manufactured with as low a series resistance as possible. Voltage power suppHes +V and -V are provided at plus and minus 24 volts respectively with a low output source impedance. The supply voltage is high enough to aUow for Unes losses and to provide for a large number of instruments or users on the line. On the other hand, the voltage also remains low enough to avoid electrical safety problems such as shock, wire damage due to shorting and other high voltage hazards. The line power source of Figure 20 is combined with a station unit line power interface diagrammaticaUy depicted in Figure 27. Station unit line interface, generaUy denoted by reference numeral 504 is comprised of three elements, a line signal interface as described above in connection with Figure 20, a diode bridge interface 506 and a li power interface 520. Diode bridge 506 is coupled to lines 501 and 503 to ensure trouble fre connection no matter what voltage polarity is connected to the input teπninals of t diode bridge. Nodes 501 and 503 conespond to nodes 500 and 502 except their ord may be reversed, so the voltage polarity is known for nodes 500 and 502 and not know for nodes 501 and 503. For example, if terminal 501 is positive and terminal 50 negative, then diodes 510 and 512 conduct, making node 516 positive and node 51 negative. On the other hand if input terminals 501 and 503 are reversed so that termin 501 is negative and terminal 503 positive, then diodes 508 and 514 conduct, agai making node 516 positive and node 518 negative. Therefore, node 516 is always positiv and node 518 always negative no matter how station unit line interface 504 may b connected to the two-wire line. line power interface circuit, generaUy denoted by reference numeral 520 i Figure 27, includes conventional adjustable voltage regulators 522 and 524 which ar used as the basis for a common constant cunent source as described below. Adjustabl voltage regulator 522 is connected to the positive node 516 while adjustable voltag regulator 524 has its input coupled to the negative voltage node 518. A constant curre source is comprised of adjustable output negative voltage regulator 524 and resistor 52 This assures that a constant cunent wiU flow originaUy from node 528 through regulat 524 into node 518. In the iUustrated embodiment, the current flowing through this pat to node 518 is approximately 1.25 volts divided by the resistance value of resistor 526 in the Ulusttated embodiment approximately 45 milh ' amps. The current value is set choice of the resistance of resistor 526 to the value of the worst case cunent-sinki demand required by loading on the negative power supply terminal 530, DVSS, an terminal 532, AVSS, or to the value of the worst case current-sourcing deman required by the loading on the positive supply terminals 534, DVDD and 536, AVD whichever is the greater magnitude. SimUarly, an adjustable constant cunent source is formed with positive regulat 522, resistor 538 and FET 540. The positive adjustable constant cunent source ensur that the adjusted constant current wiU flow from node 516 through regulator 522 int node 542. For example, in the Ulusttated embodiment, the cunent through regulat 522 is approximately 1.25 volts divided by the effective resistance provided by th paraUel resistance through resistor 538 and N-type depletion mode JFET 540. I general, the minimum attainable cunent, namely 1.25 divided by the resistance resistor 538, is several percent less than the cunent setting through negative voltag regulator 524, thereby aUowing the circuit to adjust via the voltage on node 544 to reasonable range of cunent above and below the cunent set through regulator 524. Identical zener diodes 546 and 548 are coupled to the outputs of the positive and negative constant cunent sources and between nodes 528 and 542 to set and regulate the power supply voltages DVDD, AVDD, DVSS and AVSS as referenced to ground, as long as the load cunent demand on each of these terminais 530-536 does not exceed the setting on these current sources and thus the voltage suppHes wiU remain in regulation. For example, in the Ulusttated embodiment, the zener voltage for diodes 546 and 548 are selected to ensure a plus five volts at node 542 and a minus five volts at node 528. Zener diodes 546 and 548 act to shunt any excess cunent supphed by the positive or negative cunent sources to ground that is not consumed by the load circuit. Resistors 550 and 552 are also coupled in series between nodes 516 and 518 to form a voltage divider. Resistor 554 is coupled to their junction and thence to a floating ground, GND. These three resistors serve to set a reference point for ground somewhere close to the center point between the raw supply voltages presented from the line terminals 501 and 503 through the diode bridge described in connection with Figure 20. For good system performance, a supply ground, GND, should be close to the center point between the voltages on nodes 516 and 518 which then requires the resistance of resistors 550 and 552 to be approximately equal. Resistor 554 provides a voltage on node 556 indicative of the offset of the ground from the true center point as weU as offering a path for any temporary unbalance of positive and negative cunent sources. Operational ampHfier 558, having one input coupled to node 556, in combination with resistor 595 and 560 and capacitor 562 provide a low frequency, steady state, noninverting voltage gain stage ampHfier from the voltage offset point on node 556 to the output of the amplifier at node 544. The gain in operational amplifier 558 is approximately equal to the ratio of the resistance of resistor 595 to 560. Response of the gain stage amplifier is further determined by the time constant provided by the product of the resistance of resistor 560 and capacitor 562. In the Ulusttated embodiment, resistor 595 is 1 kUohm, resistor 560 1 megaohm and capacitor 562 is 0.47 microfarads so that the time constant of operational amplifier 558 is 0.47 seconds with a gain of 1,000. The noninverting voltage gain stage provided by operational amplifier 558 in response to any significant positive or negative center point offset voltage on node 556 serves to present a voltage on node 544 which slowly rises or decreases. The voltage on node 544 relatively increases or decreases the resistance provided by JFET 540 and therefore adjusts the positive adjustable constant cunent source, comprised of the combination of regulator 522, resistor 538 and FET 540, to provide more or less positive supply current respectively for centering the supply reference point, GND, approximately to center point between the voltages presented on nodes 516 and 518 and to do so within an inaudible frequency domain with a time constant of approximately half a second. For example, a more negative voltage on node 556 wiU present a more negativ voltage on node 544 thereby raising the on-resistance of FET 540. As a result, th cunent supphed by the positive cunent source is reduced, bringing it more into balanc with a negative cunent source and requiring less current to flow from ground throug resistors 554 and 552. This drops the voltage magnitude on node 556 and serves t center the reference node GND to the voltage center point between the voltage level presented at node 516 and 518 by the line. The Final effect of operation of line power interface 520 is to present a constan cunent load to the line, 501 and 503, as weU as a constant voltage source appHed to th plus and minus five volt nodes 542 and 528 respectively with a ground reference, GND close to center point of the line voltages and to do so without injecting audible noise o the line. Line power interface may also be combined with an RC coupling networ denoted by reference numeral 562. RC coupling network 562 is a network of RC filter in which terminal 536 is provided with the RC filter comprised of resistor 564 an capacitors 566. Terminal 534 in turn is provided with an RC filter comprised of resisto 568 in combination with capacitor 570. likewise, terminal 532 is provided with an R filter comprised of resistor 572 and paraUel capacitors 574. Terminal 530 in turn i coupled to an RC network comprised of resistor 576 and capacitor 578. Also associate with terminals 534 and 530 is the RC filter comprised of resistors 568 and 576 an capacitor 580 along with power supply tank capacitors 582. Each of these RC filters are used to decouple high frequency as weU as audi frequency noise from each of the other terminals and to maintain a low nois environment to prevent cunent regulators 522 and 524 from responding to the noise and thereby injecting the noise into the line 501 and 503. The regulated terminals 501 and 503 are also coupled to a line signal interfac circuit, generaUy denoted by reference numeral 563 and described in detaU above i connection with Figures 18 and 19. The adapter box line interface is schematicaUy depicted in Figure 28 and i denoted by reference numeral 582. Adapter box interface 582 is a separate powerin circuit comprised of three elements, namely the line signal interface 563 as described i connection with Figures 18 and 19, a diode bridge interface generaUy denoted b reference numeral 584 similar to diode bridge interface 506 described in connectio with Figure 27, and a line load interface generaUy denoted by reference numeral 586 Diode bridge interface 584 is required because of the use of polarized electrolyti capacitors in the signal interface mechanism depicted in Figure 28. line load interface 586 is comprised of a smaU constant current source whic includes a regulator 588 and resistor 591 in combination' with a load resistor 592. Loa resister 592 is used as a loading device along with ground-point-resistor-setting circuit 590 to provide enough cunent drain to properly forward bias diode bridge 584 and to provide proper polarity to ensure protection to the electrolytic capacitors coupling diode bridge 584 to line signal interface circuit 563. line signal interface circuit 563 is described in connection with Figures 18 and 19. The operation of adapter box line interface 582 is as foUows. The line input is coupled to terminals 501 and 503 which are rectified by the four diodes in diode bridge 584 to provide the proper polarity to the system reference voltage point, GND, in simUar manner as described in connection with diodes 508-514 and resistors 550-554 in Figure 27. Regulator 588 and resistor 591 are configured as a positive constant cunent source supplying load resistor 592. The cunent from the positive constant current source is approximately 1.25 volts divided by the resistance of resistor 591 and is set to a current which wiU ensure linear region operation within diode bridge 584 under aU worst case conditions of signal levels on the external line or line signal interface. The cunent setting is no greater than is needed and load resistor 592 is no less than a value required to perform the necessary diode biasing in order to minimize system power. The operation of the circuits in Figures 27 and 28 now having been described, it must be understood that the line powering interface is shown only by way of example. The prefened embodiment uses balanced elements and close centering of the suppHes to the source. However, offset centering and different supply voltages other than plus or minus 5 volts can be specified and achieved as weU as providing multiple voltage sources with varying levels without departing from the scope of the invention. Different types of voltage dependent resistors other than a depletion mode JFET, like JFET 540, can also be utilized for simUar results. The invention should not be construed as requiring this type of voltage dependent resistive element. Many different approaches for generation buffering of node 544 in Figure 27 can also be used other than the voltage foUower voltage gain ampHfier which is iUustrated. StiU further, many other types of constant cunent sources can be used in place of the set negative cunent provided by regulator 524 and resistor 526 and the adjustable positive cunent provided by regulator 522 with the load resistance of JFET 540 and resistor 538 with simUar effect. The invention is generaUy directed to a single fixed cunent source and a slowly adjustable cunent source with a slowly adjustable cunent having means to move the adjustable cunent into balance with the fixed cunent source for the pmposes of avoiding the need to exactly match components as weU as adjusting to system or load imbalance experienced in real appHcations. Further, many other types of voltage regulators beyond zener diodes 546 and 5 can also be substituted. Also, a variable negative current source and a fixed positi cunent source can be used to achieve a simUar effect and performance. The line power embodiments which have been iUustrated, with or without diode bridge, and with or without line signal interfacing, are particularly distinguished its unique abUity to achieve extremely low noise at subaudio frequencies in li powering and other types of powering appHcations. The iUustrated embodiments a particularly advantageous for use in appHcations where common ground connections a not physicaUy or economicaUy achievable and/or where such common grou connections exists with problems of smaU supply imbalances which would cause grou loop noise. The invention has solved these problems by providing a floating grou reference. SimUarly, many other embodiments of the adapter box line load interface Figure 28 other than that Ulusttated are contemplated as within the scope of t invention. Such other forms may use different types of constant current source devic rather than the simple regulator and resistor combination described and stUl achieve t same effect. One-Chip System Figure 29 Ulustrates some of the appHcations which can be realized with a o chip system devised according to the invention. Everything needed to provide programmably customized communication and telephone network is avaUable using on one basic chip design in the system as described above. Not only are standard telepho communications possible on a single twisted pair, but a network of telephone receiver computers, modems, telefax devices, answering machines and any other type information handling or communication device now known or later devised for use on telephone line can be networked and accommodated within the system. Many alterations and modifications may be made by those having ordinary ski in the art without departing from the spirit and scope of the invention. Therefore, must be understood that the iUustrated embodiment has been set forth only for t purposes of example and that it should not be taken as limiting the invention as define by the foUowing claims. The foUowing claims are, therefore, to be read to include n only the combination of elements which are HteraUy set forth, but aU equivale elements for performing substantiaUy the same function in substantiaUy the same way obtain substantiaUy the same result. The claims are thus to be understood to inclu what is specificaUy Ulusttated and described above, what is conceptionally equivale and also what essentiaUy incorporates the germ of the invention. In the following clai it must be understood that where "station umt" or words to similar effect are used, it is sometimes meant to include both the station unit 18 and adaptor box 36 described above, or either one of them.

Claims: We claim: 1. A telephone communication system for communication between plurality of exterior telephone lines and a single common in house two-wire li comprising: a control unit for coupling to said plurality of exterior telephone lines and f controlling communication between said plurality of exterior telephone lines and sa common two-wire line; and a plurality of station units each coupled remotely throughout the building to sa common two-wire line, each of said station units for selectively communicating with arbitrarily selected one of said exterior telephone lines under the control of said contr unit, said control unit communicating with said plurality of station units in a ti frame subdivided into a plurality of time slots, a specified portion of each time sl being reserved for communication between said control unit and each one of sa plurality of station units, all communication between each one of said station units a control unit occurring within said selected portion of said time slot within each ti frame, whereby communication on said single common two-wire line is effected witho requiring complex control hardware within said control unit or station units. 2. The system of Claim 1 wherein each of said station units can at a time communicate with each other or through said control unit to exterior telepho lines on separate AM modulated frequency channels on said common two wire line. 3. The system of Claim 2 wherein said control unit generates a referen frequency and wherein said plurality of station units are synchronized to said referen frequency using crystal controlled VCO. 4. The system of Claim 1 further comprising a control unit with plurality of modulating interface chips, each chip for servicing a separate one of sa exterior telephone lines, one of said plurality of chips serving as a master chip and t remaining ones of said chips being slaved to said master chip. 5. The system of Claim 4 wherein said control unit chip and station unit chips are identical, but operated in different modes according to their respective operational position within said system so that a one-chip system is provided. 6. The system of Claim 1 wherein said control unit is programmable and all communication features between said control unit and said plurality of stations with said exterior telephone lines is stored within said control unit so that said system may be modified by modifying said programs stored in said control unit so that said system may be more easily changed and may be remotely modified through said exterior telephone lines coupled to said control unit. 7. The systems of Claim 3 wherein said plurality of station units and control unit are synchronized with each other through phase lock synchronization to said reference frequency. 8. The system of Claim 1 wherein said control unit communicates with said plurality of station units through a digital data channel, said digital data channel having two digital channels communicated thereon in phase quadrature through amplitude modulation. 9. The system of Claim 8 wherein said digital data channel is full duplex and wherein digital data communicated through a portion of said time frame, the remaining portion of said time frame being used for communication of control information between said control unit and said plurality of station units. 10. The system of Claim 2 wherein said control unit and plurality of station units encode, generate and communicate control and status information onto said common two wire line. 11. The system of Claim 1 wherein a plurality of voice channels a simultaneously communicated on said common two wire line and wherein said cont unit comprises means for programmably vectoring a selection of said voice channels i order to selectively network said plurality of voice channels among said station uni coupled to said single common two wire line. 12. The system of Claim 1 wherein said control unit and plurality station units each further comprise a controllable input/output port programmab capable of logic switching and sensing, keyboard and display control. 13. The system of Claim 1 wherein said control unit and plurality station units each comprise means for interfacing with a general purpos microprocessor for bidirectional exchange of data, control and status information wit said microprocessor. 14. A bus oriented processor comprising: a timing and control bus; a data bus having fixed time slot access with respect to devices peripheral to sai processor, each peripheral device being accessed through a fixed and unique portion a timing frame on said data bus; and a universal logic unit coupled to said data bus, said universal logic unit f performing a predetermined number of operations on data read from or written to sai data bus during fixed time slots within said timing frame. 15. The processor of Claim 14 further comprising: a plurality of registers coupled to said data bus, each of said registers being rea from or written into during selected and fixed ones of said time slots within said timi frame; and a corresponding plurality of combination and logic circuits, each combinati and logic circuit being coupled to one of said registers and to said universal logic un for receiving a compare/carry control signal, CMPCRY, indicative of whether or not a compare or carry occurred or not during a corresponding operation within said universal logic unit. 16. The processor of Claim 14 further comprising a plurality of bit memory means for storing machine state flags and a corresponding plurality of combination and logic circuits, said combination and logic circuits being coupled to said timing and control bus, a logical combination being detected by said combination and logic circuit from said timing bus and machine state flags being stored within said bit memory means for generating a condition and logic signal for use in other ones of said combination and logic circuits. 17. The processor of Claim 15 further comprising a plurality of bit memoiy means for storing machine state flags and corresponding plurality of combination and logic circuits, said combination and logic circuits being coupled to said timing and control bus, a logical combination being detected by said combination and logic circuit from said timing bus and machine state flags being stored within said bit memory means for generating a condition and logic signal for use in other ones of said combination and logic circuits. 18. The processor of Claim 14 wherein said universal logic unit performs only two basic types of instructions, a first instruction being a compare instruction and a second instruction being an incrementation/decrementation instruction. 19. The processor of Claim 14 wherein each time slot is comprised of dedicated cycles, each cycle within each time slot dedicated for bus precharge, selective data transfer to said bus, and selective data transfer from said bus in fixed sequential order so that bus contention is avoided. 20. A method of operating a processor to which an instruction is communicated comprising the steps of repetitively performing a sequence of timed steps, said steps comprising: precharging a data bus a first time; selectively reading a first data signal from said data bus during a first dedicate time cycle depending upon said instruction; precharging said data bus a second time; selectively reading a second data signal on said data bus during a secon dedicated time cycle according to said instruction; selectively performing one of a predetermined number of operations on said firs and/or second data signal during a third dedicated time cycle according to sai instruction; selectively writing said first data signal during said second and beginning of sai third dedicated time cycle according to said instruction; and selectively generating a logic signal, CMPCRY, during said third dedicated tim cycle according to said instruction and according to the results of said step o performing to control said step of writing said data into a register. 21. The method of Claim 20 where said step of performing sai instruction consists of one of the following groups of steps: a) comparing said first and second data signals and selectively incrementing said first signal depending on said step of comparing; b) comparing said first and second data signals and selectively decrementing said first data signal depending on said step of comparing; c) incrementing said first data signal; d) decrementing said first data signal. 22. An improvement in a method for communicating over a single two wire cable with a plurality of stations comprising the steps of: bidirectionaUy communicating with said plurality of stations in a sequence o time frames, each said time frame divided into a plurality of time slots corresponding t the said plurality of stations, each of said time slots being dedicated for communication to a specified one of said plurality of stations, said step of bidirectionaUy communicating comprising the steps of communicating serial digital data in at least one portion of said time slot and communicating control signaling data in at least another portion of said time slot, each of said time frames having a frame sync included therein to which said station units synchronize communication on said two-wire cable, whereby efficient data signaling with said station units is performed without substantial dependence on system activity. 23. The improvement of Claim 22 further comprising the step of communicating a synchronization signal to said station units from a control unit as a single timing signal against which said time frames of each of said station units are synchronized by means of said frame sync. 24. The improvement of Claim 22 further comprising the step of performing an echo back communication protocol for each station unit within its corresponding dedicated time slots in consecutive frames, said echo back communication protocol being determined by detected communication errors and selectively correcting said communication errors. 25. The improvement of Claim 22 where in said step of bidirectionaUy communicating said control signaling data is correlated with a specified register within said station unit receiving/transmitting said communication so that said bidirectional communication is easily implemented in hardware. 26. The improvement of Claim 22 where in said step of bidirectionaUy communicating each time slot is comprised of three byte cycles, two of said cycles being dedicated to serial data and the third of said cycles being dedicated to control signaling data. 27. The improvement of Claim 22 where in said step of bidirectiona communicating each of said time slots is divided into a sequence of dedicated by cycles, each of said cycles being dedicated to distinguishable groups of control signalin data relating to communication of said control unit with said station unit. 28. The improvement of Claim 27 where in said step of bidirectionaU communicating said control signaling data is directed to separate registers within sai station unit. 29. A circuit for communicating over a single two-wire cable with pluraUty of stations comprising: means for bidirectionaUy communicating with said plurality of stations in sequence of time frames, each said time frame divided into a pluraUty of time slot corresponding to the said pluraUty of stations, each of said time slots being dedicate for communication of control signaling data to a specified one of said pluraUty stations, said means for bidirectionaUy communicating comprising: means for communicating serial digital data in at least one portion of said time slo and means for communicating control signaling data in at least another portion of said tim slot, each of said time frames having a frame sync included therein to which said statio units synchronize communication on said two-wire cable, whereby efficient data signaling with said station units is performed witho substantial dependence on system activity. 30. The circuit of Claim 29 wherein said means for bidirectionaU communicating comprises a control unit and means for communicating synchronization signal to said station units from said control unit as a single timin signal against which said time frames of each of said station units are synchronized means of said frame sync. 31. The circuit of Claim 29 wherein said means for bidirectionaUy communicating communicates according to an echo back communication protocol with each station unit within its corresponding dedicated time slots in consecutive frames, said echo back communication protocol being determined by detected communication errors and selectively correcting said communication errors. 32. The circuit of Claim 29 wherein said means for bidirectionaUy communicating comprises a control unit, and at least one station unit having a plurality of registers, and wherein said control signaling data is correlated by said bidirectional communication means with a specified one of said registers within said station unit receiving said communication so that said bidirectional communication is easUy implemented in hardware. 33. The circuit of Claim 29 wherein said means for bidirectionaUy communicatmg functions so that each time slot is comprised of three byte cycles, two of said cycles being dedicated to serial data and the third of said cycles being dedicated to control signaling data. 34. The circuit of Claim 29 wherein said means for bidirectionaUy communicating comprises a control unit, and wherein said means for bidirectionaUy communicating functions so that each of said time slots is divided into a sequence of dedicated byte cycles, each of said cycles being dedicated to distinguishable groups of data relating to communication between said control unit with said station unit. 35. The circuit of Claim 34 wherein said distinguishable groups of data relating to communication between said control unit with said station unit comprise control signaling data and serial data, said control signaling data being directed to separate registers within said station unit. 36. An integrated circuit, crystal controUed voltage controUed oscillato comprising: a pluraUty of phase shifting amplifiers; a crystal controUed feedback loop coupled across said plurality of phase shiftin amplifiers to form in combination therewith an osciUator, wherein said phase shiftin amplifiers are voltage controUed; and a bias tracking means for maintaining constant bias voltage within said pluralit of phase shifting amplifiers regardless of any control voltage appUed to said phas shifting amplifiers, whereby control of output frequency of said voltage control osciUator i maintained over an extended range without saturation of said phase shift amplifiers. 37. The voltage controUed oscUlator of Claim 36 wherein each phas shifting amplifier is comprised of a CMOS inverter having an output node, capacitance coupled to said output node and a voltage controUed output impedanc modulating means for modulating the impedance of said output node and phase shift a said output node. 38. The voltage controUed oscUlator of Claim 36 wherein sai capacitance is a pair of CMOS shunt transistors coupled to said output node. 39. The voltage controUed oscUlator of Claim 36 wherein said pluraUty o phase shifting amplifiers comprises three CMOS inverters, each having an output nod said CMOS inverters coupled serially with each other to form a cascaded chain of first, second and third inverter; two pairs of CMOS shunt transistors comprising a capacitor, one pair of sai CMOS shunt transistors coupled to each said corresponding output node of said firs and second inverters; and two CMOS voltage controUed modulating means coupled to said output nodes said first and second inverters and for modulating the dynamic impedance of sai output nodes of said first and second inverters to control the phase shift of the voltage at said output of said third inverter, said output of said third inverter being coupled to said crystal controUed feedback loop so that the resonant frequency of said crystal controUed feedback loop is in turn controUed by said phase shift at said output node of said third inverter, said voltage controUed oscUlator having a fundamental frequency determined by a voltage signal appUed to said CMOS impedance modulating means. 40. The voltage controUed oscUlator of Claim 39 wherein said voltage controUed osciUator is fabricated in a CMOS integrated circuit so that said first and second inverters and said two corresponding CMOS impedance modulating means have operational characteristics dependent upon integrated circuit process parameters and wherein said two corresponding CMOS shunt transistors are coupled to said corresponding first and second output nodes to produce excess phase lag to cancel at least in part the effect of said integrated process parameters upon said operating characteristics. 41. The voltage control osciUator of Claim 39 fabricated in a CMOS integrated circuit wherein said bias tracking means comprises: a matched dummy means matching in operational characteristics said two CMOS output impedance modulating means, said dummy means having a dummy node; and control means coupled to said control voltage appUed to said voltage controUed osciUator, said control means for driving said dummy means to maintain said dummy node free of displacement current as said control voltage changes, said control means being coupled to said CMOS voltage controUed modulating means to essentiaUy maintain said output nodes of said first and second inverters free of displacement currents as said voltage control signal changes, so that self-bias of said phase shifting amplifiers is substantiaUy independent of said control voltage. 42. The voltage controUed oscUlator of Claim 41 wherein said three inverters are matched and wherein said control means comprises: two dummy inverters each matching said three matched inverters; and an operational amplifier, each of said matched dummy inverters within sa control means coupled to one of two inputs of said operational amplifier, said dum output node being fed back to one of said inputs of said operational amplifier maintain said dummy node at a net zero displacement current by driving said dum means with an output of said operational amplifier to maintain said node at a net ze displacement current while said control voltage is appUed to said dummy means, sa output of said operational amplifier being coupled to each of said CMOS volta controUed modulating means to similarly drive each of said CMOS voltage controll modulating means to mamtain said corresponding first and second output nodes at net zero displacement current. 43. A method for operating a voltage crystal controUed oscUlator in CMOS integrated circuit comprising the steps of: receiving an input signal at an input node; phase shifting said input signal to generate a phase shifted output at an outp node by means of voltage controUed phase shifting circuits; feeding back said phase shifted output signal from said output node to a cryst controUed resonant circuit; feeding back the output of said crystal controlled resonant circuit to said inp node; repeating each of said prior steps to create a regenerative osciUator, t fundamental frequency of said oscUlator being determined by said crystal controU feedback circuit; modifying said phase shift at said step of phase shifting with a voltage cont signal to change said fundamental frequency of oscillation; and maintaining the DC bias of said phase shifting circuits used in said step of pha shifting as said voltage control signal changes to extend the range of voltage contr adjustment of said fundamental frequency of said osciUator without saturating sa phase shifting circuits. 44. The method of Claim 43 where said step of maintaining self-bias comprises the steps of: applying said voltage control signal to a dummy circuit matching said phase shifting circuit; generating a gate control signal which when appUed to said dummy circuit maintains self-bias of said dummy circuit independent of said voltage control signal; and applying said gate control signal to said voltage controUed phase shifting circuits within said voltage control osciUator. 45. The method of Claim 43 further comprising the step of increasing the control range from said phase shifting circuits by applying to said output CMOS shunt capacitive transistors having simUar device parameters as CMOS transistors within said phase shifting circuits. 46. The method of Claim 43 further comprising the step of increasing the degree of phase shift during said step of phase shifting in said voltage controUed oscUlator by providing two cascaded phase shifting voltage controUed circuits driven in paraUel by said voltage control signal and coupled in series to provide an excess phase shifting lag within said voltage controUed oscUlator of approximately 90 degrees. 47. A circuit for driving a pluraUty of Ught sources comprising: a series circuit of said pluraUty of Ught sources; a pluraUty of switches in series circuit, each one of said switches coupled in paraUel across a corresponding one of said pluraUty of Ught sources so that said series circuit of Ught sources and series circuit of switches together coUectively comprise a ladder network; and a constant current source coupled in series with said ladder network, whereby said pluraUty of Ught sources are efficiently driven with a substantiaUy constant Ught emission intensity with a reduced maximum energy. 48. The circuit of Claim 47 wherein each of said switches is a logic swit 49. The circuit of Claim 47 wherein, except for said Ught sources, s circuit is fabricated in an integrated circuit. 50. The circuit of Claim 47 and said Ught sources are Ught emitti diodes and further comprising a plurality of said series circuits of Ught emitting dio and corresponding pluraUty of said series circuits of switches to form a correspondi pluraUty of ladder networks, each of said ladder networks coupled in parallel a further comprising gate control means coupled to each said ladder network selectively coupling said corresponding ladder network in paraUel to said const current source, said gate control means for selectively switching in sequence each said ladder networks with an overlapping timing so that at no time during s sequence is said constant current source ever completely disconnected from at least o of said pluraUty of ladder networks without disruption of the constant current p with its resulting noise consequences. 51. A method of driving a pluraUty of Ught emitting diodes comprisi the steps of: providing a current to said plurality of Ught emitting diodes in series circuit; shunting said current through a switched shunt around selected ones of s diodes which are determined to be in a temporarily off, nonemitting condition; and maintaining said current flowing through said series circuit of Ught emitti diodes and shunts constant, whereby a pluraUty of Ught emitting diodes are driven at low power consumpti and low noise. 52. The method of Claim 51 where said steps of providing a current and shunting said current around selected ones of said Ught emitting diodes further comprises the steps of providing a current through at least one of a plurality of series circuits of Ught emitting diodes and controlling said current through each one of said pluraUty of series circuits of Ught emitting diodes and corresponding shunts to selectively stop and start said current there through while simultaneously maintaining current through at least one of said series circuits of LEDs and shunts at all times, so that said series circuits of LEDs and shunts may be driven at different duty cycles without current spikes being created. 53. A method for compensating for line delay variations in a telephone communication system having a single shared communication line comprising the steps of: initializing communication on said single line between a control unit and at least one of a pluraUty of station units coupled by means of said line, said communication being initialized to a predetermined delay value for transmit advance time, said delay value being used within said control unit to model time delays caused by line delay between said control unit and said at least one station unit; and automaticaUy adjusting said predetermined value for said line delay according to actual line delays sensed on said single communication line, whereby changes in Une delay within said telephone system are automaticaUy accommodated. 54. The method Claim 53 where said step of automaticaUy adjusting said Une delay further comprises the step of automaticaUy measuring capacitance delay effect of said single communication line and computing a new transmit advance time to more nearly compensate for line delay with said at least one station unit. 55. The method of Claim 53 wherein said step of automaticaUy adjusting said predetermined transmit advance is performed for each of said pluraUty of station units coupled to said control unit through said single communication line, a separate automatic adjustment being made for each station unit. 56. The method of Claim 54 wherein said step of automaticaUy adjusti said predetermined transmit advance is performed for each of said pluraUty of statio units coupled to said control unit through said single communication line, a separa automatic adjustment being made for each station unit. 57. The method of Claim 55 wherein said transmit advance time determined by using a static optimal adjustment which does not vary depending on t communications being performed. 58. The method of Claim 55 wherein said transmit advance is determine by using a dynamic setting depending upon the communication task at hand for eac said station unit which varies depending on the communications being performed. 59. A circuit for reducing echo back of transmitted signals on communication Une comprising: a summing means coupled to said communication line for a bidirection communication of signals therewith, said summing means having a summing node; a balance network means for coupling a selected impedance to said summin node; and a processor means for receiving a signal from said summing node and f generating a control signal to said balance network means to select an impedance to coupled to said summing node to reduce said signal received from said summing no by said processor means, said summing node having coupled thereto signals which a measures of the transmitted and received signals on said communication line, whereby an echo of a transmitted signal placed on said communication line substantiaUy reduced. 60. The circuit of Claim 59 wherein said balance network mea comprises a pluraUty of selectively switched impedance elements. 61. The circuit of Claim 60 wherein said switched impedance elements are comprised of a plurality of switched capacitive elements and switched resistive elements. 62. The circuit of Claim 61 where said switched resistive element comprises a switched capacitor equivalent of switched resistors. 63. The circuit of Claim 59 wherein said processor means comprises: a peak detector means for generating a constant analog voltage corresponding to an amplified signal from said summing node and from said echo back of transmitte signals from said communication line; an analog-to-digital converter for converting said constant analog voltage to digital signal; and a digital processor for receiving said digital signal and generating control signal to be coupled to said balance network means for altering said impedance of sai balance network means to minimize said echo back signal. 64. The circuit of Claim 59 further comprising test tone signal generatio means for selectively coupling a predetermined test tone on said communication lin and into said summing node to generate from said communication line said echo bac signals coupled to said summing node. 65. The circuit of Claim 59 further comprising a station unit coupled t said communication Une, said control unit communicating with said station unit acros said communication Une, said station unit similarly being provided with a summin means and balance network means, said processor means corresponding to said contro unit for further communicating control signals to said balance network mean corresponding to said station unit to substantiaUy reduce said echo back of transmitte signals at said station unit as determined by optimal reduction of said echo back o transmitted signals by said processor means and corresponding balance network mea at said control unit. 66. A system for coupling signals on a two-wire communication line to pluraUty of sources/receivers comprising: a plurality of voltage-to-current amplifiers, each capacitively coupled to sai communication Une, said voltage-to-current amplifier for bidirectiona communicating between said communication line and one of said pluraUty sources/receivers; and means for providing high impedance between said pluraUty of voltage-to-curre amplifiers and said communication line, whereby said plurality of sources/receivers are coupled to said communicatio line without being characterized by mutual inductance, leakage inductance, inhere resonance, nonlinearity of magnetic media, or a winding resistance characteristic transformer coupling. 67. The system of Claim 66 wherein each said voltage-to-curre amplifier comprises two differential amplifiers, each differential amplifier having a output capacitively coupled to one wire of said two-wire communication Une comprisin said two-wire communication Une, one of said two differential amplifiers being operate as a current source into ground and the other one of said differential amplifiers bein operated as a current source from ground and both operating in a complementar fashion to each other relative to signals on said two-wire communication Une. 68. The system of Claim 67 further comprising an RC couple differential receiving amplifier having two inputs, each of said inputs coupled to one said two lines of said two-wire communication Une, said differential amplifier having a output characterized by substantial common mode rejection of signals received by sai two-wire communication line. 69. The system of Claim 68 further comprising a pluraUty of echo balance means for canceling echo back signals and wherein said RC coupled differential amplifier has its output coupled to each of said echo balance means, one of said echo balance means being provided for each of said voltage-to-current amplifiers so that the echo back signal for each source/receiver is substantiaUy reduced. 70. The system of Claim 66 further comprising a balanced DC load means for allowing line powering over said communication line without substantiaUy lowering the AC impedance of each said voltage-to-current amplifier and without disturbing signal current information. 71. The system of Claim 66 further comprising a balance load ground means for setting a ground reference while maintaining high impedance of said communication line and to aUow said plurality of voltage-to-current amplifiers to be coupled to said communication line without significantly lowering the impedance of said communication Une. 72. A telephone Une interface for coupUng central office telephone lines and equipment to a pluraUty of devices comprising: a transformer having its input coupled to said central office telephone lines and having an output; a load resistor coupled to said output of said transformer; an output amplifier for receiving an audio band signal from said plurality of devices; an inverting amplifier coupled to said output amplifier for inverting the output of said output amplifier and coupling said inverted output to said load resistor; an input amplifier coupled to the output of said transformer for generating an input signal to said pluraUty of devices; and a balance network coupled between the output of said output amplifier and the input of said input amplifier for summing said audio signal with said inverted audio signal output through said transformer, wherein said balance network has a impedance simulative of said central office lines and equipment, said summi performed at said input of said input amplifier so that echo back signals a substantiaUy reduced. 73. The telephone Une interface unit of Claim 72 wherein said balan network further simulates the output impedance of said transformer as presented said input amplifier when said transformer is coupled to a telephone central office U and equipment. 74. The telephone Une interface unit of Claim 73 wherein said balan network comprises means for providing a device resistance equivalent to said telepho central office. 75. The telephone line interface unit of Claim 73 further comprisin means for providing an equivalent impedance to a typical transmission telephone Une. 76. The telephone line interface of Claim 73 further comprising means for compensating for the magnetizing inductance of said transformer. 77. The telephone line interface unit of Claim 73 further comprisin means for compensating for said load resistance. 78. The telephone line interface unit of Claim 73 wherein said balan network comprises means for compensating for winding resistance of said transform as it effects low frequency gain of said input amplifier. 79. The telephone line amplifier unit of Claim 73 wherein said balance network comprises means for compensating for low frequency cut off associated with magnetizing inductance of said transformer. 80. A circuit for driving a piezo element comprising: a first CMOS driver for generating a driving voltage in response to an input control voltage, said driving voltage being coupled to one of two inputs to said piezo element; a second CMOS driver for generating a driving voltage in response to an input control voltage, said driving voltage being coupled to the other one of two inputs to said piezo element; and means for generating two distinct input control voltages opposite in phase, one of said control voltage being coupled to control said first CMOS driver and the other one of said control voltages being coupled to said second CMOS driver, whereby an increased voltage amplitude swing greater than the magnitude of the supply voltages across said first and second CMOS drivers is applied across said piezo element. 81. The circuit of Claim 80 wherein each of said drivers is a CMOS driver having an output voltage swing equal to the difference of the positive and negative supply voltages, Vp - Vn. 82. The circuit of Claim 81 wherein each of said CMOS drivers is comprised of a first stage CMOS input driver having an input coupled to said control voltage, and a second stage output CMOS driver, said second stage output CMOS driver having an input coupled to the output of said first stage output CMOS driver, said second stage output CMOS driver generating an output voltage swing equal to the difference in voltage supply across said second stage output driver, Vp - Vn. 83. A method for generating an improved audio output from a piez element comprising the steps of: providing an input signal having a frequency at which said piezo element is to b driven; inverting said input signal; coupling said inverted input signal to a first CMOS driver; generating an output signal from said first CMOS driver in phase with sai inverted input signal coupled to said first CMOS driver, said generated output signa having an amplitude equal to the difference in supply voltage coupled across said firs CMOS driver, Vp - Vn; coupling said input signal to a second CMOS driver; generating an output signal from said second CMOS driver in phase with sai input signal, said output signal generated from said second CMOS driver having a amplitude equal to the difference in voltage supply across said CMOS driver, Vp - Vn and coupling said generated output signals from said first and second CMOS driver across said piezo element, whereby a voltage swing equal to substantiaUy twice the voltage difference acros said CMOS driver is appUed across said piezo element to generate increased audi volume. 84. In an amplifier having an input and output, a circuit for voltag limiting the output of said amplifier comprising: a first series circuit of bipolar transistors coupled across said input and outpu of said amplifier, said first circuit of bipolar transistors being arranged and configure to limit the negative output voltage of said amplifier; and a second series of bipolar transformers coupled across said input and output o said amplifier, said second series of bipolar transistors being arranged and configure to limit the positive output voltage of said amplifier, whereby voltage limitation of the output of said amplifier is achieved while maintaining high linearity within the useful dynamic range of said amplifier. 85. The circuit of Claim 84 wherein said first and second series of bipolar transistors are comprised of at least one bipolar transistor having a base, collector and emitter, wherein said base and collector of said transistor in said series circuit are coupled in common as the input of said transistor. 86. The circuit of Claim 85 wherein of each said series circuits is comprised of at least two transistors, said at least two transistors being defined as an input transistor at one end of said series circuit and an output transistor at an opposing end of said series circuit, and wherein each said transistor has said emitter coupled to said base and coUector of an adjacent one of said transistors in said series, and wherein said output transistor is coupled to said input of said amplifier and said input transistors is coupled to said output of said amplifier. 87. A circuit for providing at least one dual voltage supply on an output comprising: a two-wire communication/power source and supply line having a positive and negative voltage on said two-wire line; and floating ground line power supply means for providing a floating reference center voltage for said voltage supply to regulate power suppUed on said Une and isolate said Une from ground loop noise. 88. The circuit of Claim 87 wherein said floating ground line power supply means comprises: fixed current source means for providing a constant current source having its input coupled to said line and having an output coupled to said output of said supply voltage; and adjustable current source means coupled to said line and to said outp of said supply voltage, wherein said adjustable current source means is slowly varied balance said fixed current source means to approximately maintain said floati reference center voltage centered between said positive and negative voltages on sa two-wire line. 89. The circuit of claim 88 wherein said adjustable current sour means is varied at a rate which is subaudible. 90. The circuit of Claim 88 wherein said fixed current source comprised of a voltage regulator and a series output load resistor and wherein sai adjustable current source means is comprised of a voltage regulator and a variab resistance device. 91. The circuit of Claim 90 wherein floating ground line power supp means further comprises a voltage divider and a gain stage amplifier, said gain sta amplifier having an input coupled to said voltage divider and an output coupled to sai variable resistance device, said voltage divider being coupled between said positive an negative voltages on said two-wire line. 92. The circuit of Claim 87 further comprising a diode bridge, sai diode bridge being coupled between said two-wire line and said floating ground U power supply means, said diode bridge for providing a fixed polarity to said floati ground line power supply means regardless of the nature of coupling between said dio bridge and said line. 93. The circuit of Claim 86 wherein said circuit is used in combinatio with telephone station units and wherein said floating ground line power supply mea suppUes current to telephone station units. 94. The circuit of Claim 87 wherein said circuit is used in combination with non-line powered communication devices and wherein said floating ground line power supply means comprises a non-line powered interface to provide a predetermined and minimum amount of bias current with a smaU constant current load on said Une and to provide linear signaling with said communication devices through said two-wire line. 95. The circuit of Claim 94 wherein said non-line powered interface further comprises a diode bridge for providing a predetermined polarity from said line and a fixed current source and voltage divider coupled across said line, said fixed current source coupled to said voltage divider to provide said floating reference center voltage and to forward bias said diode bridge to maintain operation of said diode bridge in a linear region. 96. The telephone communication system of Claim 1 wherein communication between said station units and said control unit is on at least one AM digital channel. 97. A telephone communication system for communication between a plurality of exterior telephone lines and a single common in house two-wire Une comprising: a control unit having a chip for coupling to said pluraUty of exterior telephone lines, said chip for controlling communication between said pluraUty of exterior telephone Unes and said common two-wire line; and a pluraUty of station/adapter units having said chip, each station/adapter unit coupled remotely throughout the building to said common two-wire line, said chip in each of said station/adapter units for selectively communicating with an arbitrarily selected one of said exterior telephone Unes under the control of said chip in said control unit, wherein said chip in said control unit and station/adapter unit chips are identical, but operated in different modes according to their respective operational position within said system so that a one-chip system is provided and so that communication on said single common two-wire line is effected without requirin complex control hardware within said control unit or station/adapter units. 98. The system of Claim 97 wherein said chip in said control un communicates with said pluraUty of station/adapter units in a time frame subdivide into a pluraUty of time slots, a specified portion of each time slot being reserved fo communication between said control unit and each one of said plurality o station/adapter units, all communication between each one of said station/adapte units and control unit occurring within said selected portion of said time slot withi each time frame. 99. The system of Claim 97 further comprising voice band and digita band interface means in said chip for communication between said control unit an said remote station/adapter units. 100. The system of Claim 97 in combination with at least one compute having modem communication capabUity and/or at least one telefax device, an wherein said chip is programmable, is capable of communicating signals in telefa compatible format and in telephone tone signal format, and is accessible within sai system by extension dialing, said control unit being programmed to selectively coupl said computer and/or telefax device coupled to said chip in said station/adapter unit to at least one of said pluraUty of exterior telephone Unes through said single commo in house two-wire line. 101. The system of Claim 100 wherein a pluraUty of said computer and/or telefax devices are coupled to said station/adapter units, said control unit fo selectively coupling said plurality of said computers and/or telefax devices in programmable network. 102. The system of Claim 97 wherein said station/adaptor unit emulate a telephone receiver with respect to communication characteristics on said pluraUty exterior telephone lines. 103. The system of Claim 102 wherein said station/adaptor unit further comprising means for selectively powering said computer and/or telefax device upon being accessed. 104. The system of Claim 102 wherein said station/adaptor unit further comprising a modem and a serial input/output port for external communication. 105. The system of Claim 97 further comprising an auxUiary communication device, said auxUiary communication device having said chip for communication with said system on said single common in house two-wire line.

Inventors: KOMAREK JAMES A, LEWIS HAROLD F, MINNEY JACK L, NORDINE STEPHEN P, STOCKMAN JOHN F, WADA RICHARD, KOMAREK, JAMES, A., LEWIS, HAROLD, F., MINNEY, JACK, L., NORDINE, STEPHEN, P., STOCKMAN, JOHN, F., WADA, RICHARD
Applicants: CREATIVE INTEGRATED SYST INC, CREATIVE INTEGRATED SYSTEMS, INC.
Application year: 1994
Main classification: H04J3
IPC codes: H04J3, H04L7, H04M3, H04M9, H04M19